Other software - and even Kicad is terrible at this - require that everyone opening a file has to be using the same version, and there is no forward compatibility at all.
I agree that having "future compatibility" in KiCad would be very nice, but with an FOSS program like KiCad it is much less of an issue because you can always update to the latest and greatest stable version. There is no need to cling desperately to a 5 years old (or even older) version.
As someone who worked at a software company long ago [edit: where I worked in the support department, helping customers with our software]: yyyyeah, I'm guessing you've never dealt with much corporate IT. In some cases, they will not install software outside of a complete OS image that they ghost onto the user systems, and they do that once or twice a year. They
may (or not!

) allow Windows updates to self-install, or they push those out separately, but applications are shit out of luck.
Yes, that is absolutely as insane as it sounds. But it's the reality in some companies. At the company I worked at, this meant that some users [edit: at our customer companies, not ours] were stuck dealing with bugs that we had fixed months ago, sometimes years.
While I was there, one customer's company upgraded
to Windows Vista in 2011.

(Or was it from Win2k to XP? I don't remember the details, only that they were upgrading from prehistoric Windows to an already-obsolete Windows...)
Upshot being that just because updates are free of charge in no way means that all users can actually get said updates.
Altium's handling of file compatibility between versions is truly commendable. KiCad could really stand to do better in this regard. (As could a lot of software...)