I don't know why everybody seems so sure it's happening. Is it just because Microsoft's doing it or is there more going on?
There are a few factors.
Simplicity of the architeicture to actually program it to work.
Basic business sense regarding carrying your purchasing history forward in this time of interconnected accounts, long lasting service based gaming and PS+ subscriptions(which sony has already consistently noted in their financial reports)
And of course, regarding the competition, Sony is obviously aware of what MS has been putting out, and so at the very least, they will want to not fall behind any further on that front.
Its not even really about BC, but about retaining users in an ecosystem that is long running and is continuous.