I see what you're getting at. Not all keg beer is pasteurized, though. And many non-keg-conditioned beers are pasteurized.
And even "live beers" will "die" (flocculate), and in turn are either filtered to remove the latent yeast (as nearly all commercial brews are) or simply cooled below the yeast's preferred temperature. For most ale yeasts, that tends to be around 65˚F/18˚C, ±8˚F: some a little warmer, some cooler.
But "real ale," i.e. cask-conditioned, is brewed within very careful specifications so that the minimum remaining sugars (either from the malt itself or from added dextrose) are present to manintain natural carbonation. Too often pubs will improperly cellar these, as you say, or cellar them properly but then serve them too warm. A good pint drawn from the cask is a sweet thing. But there are so many variables: the pub itself, the barman's skill, whether your pint is an early or late draught, and how fresh the cask is. But, it's traditional, and like most things traditional, when done right, has few modern equals. Cask-conditioned beers don't ship well, and I am sure to enjoy them whenever I visit the UK.
But because of all those variables, brewers refined their brewing process to take advantage of modern kegging technologies as they evolved so that their beers would find fuller flavour through the consistency in the CO2 draught environment, even British and Irish brewers.
Either way, these do not effect the flavour imparted by the malt, hops, or even yeast. They effect the conditioning: the way the beer is carbonated and its resultant smoothness on the palate. (Why do you think Guinness insists on its proprietary nitrogen mix for its draught systems?)
Bottle-conditioned beers do keep doing their work, but will eventually stop once the yeast runs out of priming sugar (usually dextrose), which at that point the remaining non-flocculated yeast will consume voraciously before the malt. If they didn't stop, the bottles would explode, which does happen from time to time at the brewery before shipping.
Beer really isn't theory. It's four ingredients and time. A good brewer doesn't have a theory, but is aware of the mistakes of others before him and looks for ways to avoid them himself.
Clearly, the both of us love a good beer. I'm assuming you're English. I'm from Northern Ireland but live in Montana now, but my work brings me to the UK several times a year. I hope you don't mind if I pm you to meet for a pint or two. And if you're ever in the Rocky Mountains to ski, please pm me. Although, the barmen here more often than not play dumb when you ask for a better draught
. In any case, it's great talking beer.