when the PS4 would maybe cost £100 more to stick in the stuff that would enable it to play all PS3 games
If that's the case, I already said that you shouldn't be making a new console then. If the new comes at the expense of the old, then keep working on the old platform until you figure out a solution. IMO.
Another solution, as I said somewhere else in the thread, would be to digitize the previous console library (or at least the most famous titles of it) and sell them online. That way you maintain your revenue, can proceed with the next generation and maintain the historical record.
if that was priority one for the PS4 or the Xbone, whilst still keeping it at a good price point
This was my objection. If you can't maintain the old hardware, do not make new hardware. Unless you can find a way to digitize the library of the previous generation or make the new one able to run it, I believe that you should not be trying to technologically charge forward.
It's not like the past generation of games just explodes when a new one comes out, at least not for years and years.
Not explode, more gets lit on fire and slowly burns to ash. With every year that goes by, there are fewer past consoles in the world and fewer people get to experience the games that built the industry. As I said: can you imagine what English literature would look like if the entire language was re-written every ten years and nobody could understand the work of the previous cycle? Oh, sure, there'd be a few who could, but those people would lose their ability to do so over the years and everything we created in that cycle, no matter how grand or important or pioneering, would just disappear.