I think a lot of plugin devs were waiting for HPOS to be released officially before starting work on making their code compatible; it’s easier to develop against a final release rather than a moving target in development. I’m sure that the vast majority of plugins and themes will be compatible by next summer, though there will doubtless be many other plugins that aren’t compatible with HPOS and are no longer being developed. Shop owners will have to decide whether to find alternatives or update the code themselves.