I think that, with age, many people simply "grow out" of football enthusiasm. Not all, of course, but many. Football is a habit. Break it, and you simply develop another way of living.
Added to that, nowadays you can still watch most games on TV/streams/whatever. What you lose is the "match experience" and for many older fans especially that is not much of a loss, as the modern "match experience" is basically shit and a rip-off. Speaking for myself, I have only been interested in the football - not some wankpit "entertainer" droning on, or uber-loud music, or crappy lager in plastic glasses. The traditional atmosphere created by the fans is largely dead due to gentrification.
Now of course we are all different, and I am sure that there are thousands absolutely gagging to get back in the Etihad and similar grounds. The $64,000 question is will crowds recover to their former levels? I think there are so many random factors that it is impossible to predict that right now. But it's certain that for many this hiatus has broken "the habit". Will there be new fans, especially youngsters, eager to replace them? Who knows?