It's my understanding that the initial cost to get the ip taped out, stable and up and running is what's higher when producing chips on the newer processes. Once everything is running, considerable cost reductions come in at similar rates as before. Not to mention that the smaller/cooler/more power efficient chips lead to cost savings in the rest of the hardware such as board complexity/voltage regulation requirements, power supply, thermal design/cooling, even packaging size/weight/shipping. The initial outlay has to be taken into account of course, but with each chip revision/version on PS5 (og 7nm, rev 6nm, slim ?nm, Pro etc.) likely seeing scales of ~20m, there's still every reason to revise regularly. And a Pro chip isn't some crazy, impossibly expensive, out there idea.
For MS it's a lot harder. Sony have sold 70% more consoles so have the economies of scale on their side. Not to mention XSS is selling nearly 3-to-1 against XSX. Both Xbox SKUs use different chips while both of Sony's use the same chips. Also, the nature of MS' chip design for XSX likely makes scaling more complex (whether it be smaller revisions of the same or upgraded versions for midgen refreshes).
Despite all the talk of expensive chip fab, Sony have made every decision to maximise their potential for cost reduction and upgraded consoles; which paired with their market lead, puts them in a great position on this front. A Pro/Slim line up by the end of next year with respective prices of $349/$499 or $399/$549 is perfectly reasonable.