I agree that the lack of backward support is a PITA, but it is also something Apple has done for many years. You will also notice that no Apple software can ever save to an earlier format. This has been a HUGE issue with both FPM (which started life as an Apple product) and FCP.
There have been ways to get installers to prior versions. When they were still using disks you could sometimes find retail versions etc. You can also find links to downloads but I wouldn't trust any that are not actually from Apple which would be at or close to zero.
The lesson is to always download copies of the OS when you can and store them for when you need them. I got into this habit because ProTools always takes awhile to get compatible and many times by the time they certify a release Apple has already moved on.
Doing a "clean install" is not really a good idea for most users. If you want to do that say because you are running weird OS altering software then the way to do it is to set up a master disk and use that to copy from to your boot drive.
Before OSX it was something that we did periodically along with defragmenting the drives. That is really a waste of time with OSX for 90% of users.
I made sure to grab a 10.10 installer when it was available because when I do move (from 10.6.8) many of the things I need to keep working will work (I hear) on 10.10 but not (probably) on 10.11. I downloaded those installers awhile ago in anticipation.
It would be fantastic if Apple acted as a responsible developer should and made older versions available for those that need them, but that would be a BIG change in policy and I wouldn't count on it. Just as I never expect Apple to support saving to earlier versions of documents, as pretty much every other developer in the world does. It's just not their culture, "never look back".
