I have to say that it sounds disingenuous to say there are PS2 games on the service. They are PS4 ports of PS2 games so effectively are PS4 games. And that initiative has died already.
That said, a big issue is that the rack servers don't have the necessary hardware to run these older PS games. They also don't have PS5 hardware, which would explain the lack of PS5 games on PS Plus.
I don't think the lack has anything to do with Sony wanting to delay PS5 games. On the contrary, Sony were making rather good moves to PS Plus prior to covid hitting, and then things just stopped. And since they can't even manufacture enough PS5's for demand, sparing some for random servers doesn't make sense (both from hardware sales and the inevitable software sales they are seeing too).
When things stabalise I imagine we'll see the next step in PS Plus evolution. For me, as long as it streams/ downloads PS4 and PS5 games, that's all I would care about. And a smooth connection. Anything else is just icing.