All guns fired?
Also, PS5 at $449? Only if it matches or slighly outdoes One X performance. $499 is the minimum, not the ceiling.
As for the budget gamers: If the PS5 is backwards compatible, there's little reason to keep your PS4. So many will trade them in for a PS5, meaning that the used market will flow over with PS4 consoles and ruining the value of new PS4 consoles. Even at $149, they won't sell many of them anymore after PS5 launch - except if the PS5 is too expensive for everybody (Hello, PS3 launch!).
So, wait, which is it? Is the PS5 going to be so expensive that there will be a $300+ (which won't be closed much if the used market becomes as saturated as you say it will) between it and the PS4, or is it going to be cheap enough that no PS4's will be sold after it's launch because everyone, even budget gamers, have jumped ship? Can't have it both ways.
Also, launch PS3s played PS2 games, and while there were tons of used ones out there, the PS2 still sold. Same goes for the PS1. The PS4 isn't going to just stop selling. At $149, it would be closer to an impulse buy for budget gamers and parents wanting a cheap system for the kids. Those types of people will wait for the PS5 to hit that price, as well.
And it will have been 3 years since the X launched and you think Sony could only slightly outdo it with a $50 savings? That's not how tech works. Sony also always takes a loss on HW. So, while it may be priced at $449, it'll cost them $549 to build.