What I'm thinking with these upgrade units is that while the PS4 and PS4.5 are out, devs make games with the PS4 in mind, and the 4.5 owners get those games at higher resolutions and framerates. As the years go on, devs start focusing game development on the 4.5, and leaving behind the 4 owners. Sorta like how devs act when transitioning to a new hardware gen.
As this is happening, with exclusive experiences being made on the PS4.5, Sony releases the PS5, having the PS4 made obsolete, and those users migrate to the PS5. PS5 would run PS4.5 games better, and would have exclusive games by Sony/AAA to encourage users to upgrade to the PS5. PS4.5 users don't need to upgrade just yet as they know a PS5.5 will come out and they are getting most of the games the PS5 is getting, just running worse.
It'll turn into a cycle of X > X.5 > X2 > X2.5, with X being made irrelevant once X2 is out, and X1.5 being made obsolete once X2.5 is out. Still the same "cycle" of when to upgrade hardware, but now with a mid gen upgrade that you can jump into if you so choose.