Losses at Mandriva

A. First, it's not about just keeping one other version of one well known lib available, you need to repeat with all the other libs apps written about that time commonly used (and recurse for all their dependencies).

B. Second, it's not "just keeping", it's also maintaining like fixing security problems and such.

C. Then, people will ask for N-x versions too, why stop at just N-1 ?

A. + B. + C. are already a major amount of work most people don't realise the scope of.

D. But then because you have several sets of versions available application writers will start doing strange stuff like relying on era N versions of library X and era M for versions of library Y (and they'll have unlimited arguments about why they should be allowed to do that, that basically boil down to "I have a bug with version N of library Y and can't be bothered to fix it").

That will in turn increase component interdependency complexity and distribution maintenance work.

A few years later when your distribution has ground to a halt because of letting application writers not make the effort to port to latest library versions, you'll need to kill some old stuff to liberate resources, and make all the delayed porting work happen anyway. And app writers will complain bitterly you're not keeping your part of the unsustainable bargain you were a fool to accept in the first place. (because of course their PHB will have refused to allocate resources do port anything to new versions since the old versions were available, so no matter how long you keep the old versions porting to new versions will always happen at the last time in crash mode).

Also multiple versions mean:
1. more mirror load
2. more bandwidth load
3. users that complain of update time and that the distro does not fit on their netbook anymore
4. users that complain because feature A in app X using lib version N is not available in app Y using lib version M
5. new bugs because of interactions between versions that were never supposed to be used together
6. new bugs because of applications getting confused and selecting the wrong component version
7. configuration hell because all this stuff uses slightly different configuration files, that will need to be kept in sync

And I'm sure there are many other points I've forgotten. But basically once you try to do it and realise all the implications you quickly reverse the decision. This kind of setup only looks good from the point of view of the people who don't need to deal with the maintenance fallout (developers who don't want to spend time porting to new versions and users of their apps who think it's easier to badger their distributor than getting the developers do their job).