I think that's a very important factor too. MS have done great work in running older games on newer generations. I was really happy with the number of PS4Pro 30 fps modes running at 60 on the PS5.
I'd love to see Sony get their finger out and get some solid 1st party emulators running on the PS5. Other than the emulated version of the PS4's PSP emulator. As much as I'm a PlayStation guy, I really have to wonder wtf they're thinking sometimes!
They really need to get both x86 and ARM emulators up, running, and improving performance, starting with the PS1. Unpaid enthusiasts are bloody doing it Sony!
But as much as I think forwards compatibility can help reduce risk, I think plain old backwards compatibility reduces risk plenty on its own. As long as customers don't lose access to their libraries, I think most will see free upgrades (forwards compatibility) as a bonus, but not necessarily a major factor in ecosystem decisions.
I've seen some Sony 1st party games that can run at a 40fps mode on a VRR TV. That's why I'm leaning more towards the 1.5x raw power purely through clockspeeds. Any games running at native 40fps can run at 60, native 60fps can run at 90, native 90fps at an easy 120 - the latter two being particularly beneficial for VR.
And that's only in terms of easily accessible, raw power. If they're able to leverage AMD's responses to DLSS3, we may see PS5 games with 120fps modes able to entertain ~1440p resolutions. If they can also implement RDNA3's improved RT and Infinity Cache, we may see really nicely improved 40 and 60 fps mode graphics.
That'd be 3.345GHz for a PS5 Pro. A fairly tall order, but I think it's within the realm of possibility that it may be possible late next year on 5nm given existing RDNA3 GPU's have reached 3.5-4GHz in non-gaming workloads.
Heck, even squeezing out 1 extra GHz at 3.23 is still a 1.4484 times increase. Good enough overall if also applied to the CPU clockspeed and GDDR6 bandwidth, I'd say ¯\_(ツ)_/¯