I didn't say backcompat isn't important.
There are lots of other ways to run old binaries than at your main OS level.
There are tons of other platforms that precede the current ones.
I would not like the requirements from those platforms to hamper the current gen os.
I do think it's valuable to be able to run the programs from those platforms.
Most software that was ever written was done so by companies that no longer exist, or by people (not working for a software company) no longer associated with those company they wrote the tool for. In many of these cases the source is not available, so there is no way to recompile it or update it for a new platform, but the tool works as good as ever.
Binary backcompat is incredibly important.