As the tributes to Humphrey Lyttelton piled up the other month, there was the unmistakable shadow of an elephant in the room. Everybody knew it was there, and a few couldn't resist chucking a few buns in its direction. It was, of course, this: will I'm Sorry I Haven't a Clue continue?

The tactful response is that it's too early to say. To immediately cite successors to the Humph crown seems disrespectful, but to declare him irreplaceable seems equally brutal and mistimed. But the latter conclusion is, with some sadness, also the correct and inevitable one. Home truth time: without Humph, ISIHAC is fucked.

This isn't simply because Humph was good. He was good, of course, and we surely don't need to expand on why. It's not often that we're given to quoting John O'Farrell, but he did get it spot-on when he described ISIHAC as the audio equivalent of lying in a hot bath. Humph wearily reminding us of 'Una's look of surprise as she saw Nevill Shute in Lionel's face' at 6:30pm on Radio 4 is probably the very definition of joy itself. In order to understand why Humph became so good, however, we have to view how his character developed over the show's 36 years on the air.

Humph was a product of a radio/television industry which unfortunately no longer exists. Put simply, ISIHAC was allowed to be rubbish for about two decades. Check out a 1970s or 80s edition and the contrast to the 'hot bath' shows we know and love is startling. In 1991, On the Hour mercilessly parodied the show by exposing its sheer bleakness - Graeme Garden and Barry Cryer are heard self-consciously singing "Bottom bottom, counterpane..." to the tune of 'The Girl from Iapanema'. On the Hour producer Armando Iannucci has since claimed the sketch was affectionate, but this has the whiff of damage limitation about it - to most comedy fans at the time, the spoof was an accurate demonstration of ISIHAC as deeply unessential listening. It was generally viewed with the same disdain as Week Ending (also parodied by On the Hour), and few regarded the spoof as sacrilege.

What changed everything was the arrival of Jon Naismith, who produced his first ISIHAC only a few weeks after the On the Hour spoof had been broadcast. Under Naismith, the show gradually transformed into something rather wonderful: instead of simply featuring old men sitting around exchanging corny puns (which is pretty funny in itself), the show became a glorious pantomime with its own internal logic; a deeply strange parody of a game show interwoven with endless private/running jokes, drenched throughout with wave after wave of cacophonous audience laughter. Crucially, though, this transformation was not an instant one - even under Naismith, ISIHAC was given time to evolve on its own terms. The death of Willie Rushton in 1996 was a severe blow, but the show was just about strong enough to continue. Why? Because at its heart was the figure who had transformed and matured the best - Humph himself.

Again, listen to those early episodes. Humph is pretty much playing himself - a jazz bandleader who's chairing a quiz game on the side. He's amiable enough, but largely anonymous. Slowly, however, and particularly under Naimsith, the Humph 'character' took shape - the bored, disdainful headmaster figure with the acidic, beautifully-timed asides. Humph simply got better and better, and it's arguable that he was at the top of his game in the show's final years.

This is why it's futile to even consider replacing Humph. It'd be like smashing a decanter of 36-year-old crusted port and serving Ribena instead. Quite simply, Humph was good because he was allowed to become Humph slowly and naturally - with the guiding hand of a savvy producer, sure, but at his own pace nonetheless. The BBC in 2008 doesn't have that luxury any more. Everything these days must be an instant hit, or at least perceived as one.

Humph's death provoked similar feelings to the death of John Peel, in that the grief is twofold: we mourn a great man, but we also mourn the fact that such a great man could never rise to prominence today. This isn't just banal 'We'll never see his like again' thinking, but an angry thought about how radio and television is no longer the playground for mavericks it once was. Anyone slipping into Humph's shoes would have two options, neither very promising: do an ersatz version of Humph's act, or contrive a forced persona. Either way, it would have to be labelled a 'success', and immediately so.

The replacement host can't be an existing contributor like Jeremy Hardy or Stephen Fry either, because that carries its own problem - namely, there'd be too much of an audible knowing-wink in their voices. They just couldn't deliver those 'Samantha' lines in anything other than a nauseatingly gushing, over-aware, hey-everyone-I'm-in-on-the-joke way. William Franklyn had similar problems playing The Book on The Hitch Hiker's Guide to the Galaxy - he had a well-meaning stab, but he just couldn't nail Peter Jones' (naturally) bewildered tone.

Despite all these obvious obstacles, there is a type of ISIHAC fan who simply refuses to confront them. Check out any forum thread on the topic and you'll see people throwing out the names of fruity-voiced old men as if that's all there is to it: suggestions have included Oliver Postgate, Michael Aspel, John Sergeant, and - heaven help us - Jeremy Paxman. They'll settle for anyone, it seems, so long as the show continues. It's similar to the way some ISIHAC fans reacted when the team considered reducing the number of programmes per year in order to spread their material more thickly - the main response seemed to be 'Aw, that means there'll be less recordings to go to!'. Which says it all, really - there's a particular type of comedy fan who still sees the finished artwork (the transmitted show) as incidental to their own selfish desire for a great night out.

Those doing the name-throwing are working on the 'Funny bloke = funny host = funny show' principle, oblivious to ISIHAC's unique character-dynamic. After all, it's not like most comedy panel games where you can replace the chairman with any number of people so long as they can churn out the nob gags. Any replacement would inevitably sound like someone merely playing tribute to Humph - pleasant as a one-off, maybe, but it's not comedy.

Some ISIHAC fans have played the 'It was Humph's wish for the show to continue' card, but that's a flawed approach too. After all, just because Humph wanted it to continue doesn't necessarily mean it would be a good idea. Apart from anything else, what else was he going to say? 'I'm brilliant and the show cannot possibly exist without my genius. Right, here's Bad Penny Blues...'?

ISIHAC humour can, and should, continue without Humph, of course - look no further than You'll Have Had Your Tea for proof of that. There isn't a Humph-shaped hole in that series for the simple reason that it wasn't conceived with Humph as a participant in the first place. It's ISIHAC-esque in tone, but it's a totally different programme.

So rest in peace, Humph. And, if anyone's got any sense, rest in peace ISIHAC too. Although if you ask us, I reckon they should get the bloke who played the second Joey Boswell in Bread. And I hear the chap with the non-falsetto voice who took over from Jimmy Somerville in Bronski Beat is still available...