Unix was the first to start using 3 button mice as standard, almost a decade before they really showed up in Windows world, and paste on middle mouse click has always been the standard behaviour in the Unix world, so changing it now would be very weird for *nix users. Whether Linux should follow the common Unix defaults or Windows defaults for their UI/UX has been a long running debate in the community.
I would personally be really confused and annoyed if I sat down at a Linux machine and middle click didn't paste, because that is the behaviour I've been seeing for literally 25 years.