The problem with having just one (of a class preserved) is which one to preserve?
There was a thread a couple of years ago, when the 91s were on the verge of being scraped where about thirty of the thirty one were being suggested for the "National Collection" (since they all ticked various boxes - the first in service, the last in service, the first one to achieve various things...), which presumably means that the thirty first one should also have been saved for the nation because the unique thing about it was that it wasn't the first or last one to do anything
Personally, whilst I'm fairly cynical about preserving the track bed of every quaint little branch line, I don't mind if people want to preserve lots of old locomotives - even if they never get any main line use
One group preserving an old loco in Norfolk isn't really affected by whether or not there's also one in Merseyside or Somerset being preserved - you're going to have a local team of volunteers, nobody is going to travel hundreds of miles each weekend to help look after a loco on the other side of the country
That probably means we'll end up with several of some classes in preservations and virtually none of others (there are more of some tiny classes like Deltics in preservation than some larger classes but that's fine, it's up to people what they want to preserve and some classes resonate more than others)
There's also the point that some people aren't preserving something to haul trains on main lines, maybe they are preserving it just to keep a piece of their childhood together, even if it's more of a "static" display, not everything has to be preserved for practical reasons - if you can find a siding somewhere for your slice of 1960s nostalgia to sit there and rust for a few decades then good luck to you, it's not hurting anyone (and it's not as if scrapping it would miraculously keep one other member of the class alive, given that it may not have the right donor parts etc)
(I hate threads like this that make me take the "soppy" side, rather than being hard headed!)