What is hardware that enables 60 fps, and why wasn't it a feature of past consoles? Is there a CPU : GPU ratio above which 60 fps becomes more reliable? Couldn't PS2 or OXB or PS3 reach that ratio?
I'm not an expert on pc profiling, but there is a balance between cpu and gpu. You don't pair a 1080ti with an i3 cpu. You'll be cpu-limited in a lot of games, and it'll limit your frame rate. In terms of a console, there's no answer I can give you, but it's not a special technology. It's more cores, more clock, more ipc, or maybe more cache. There are games on X1X an PS4 Pro that I believe are cpu-limited and fail to maintain 60fps in their performance modes. The design would have to be based on projections. You'd have to profile a lot of games, and get feedback from developers, to get an idea of what they would need to be able to get their games up to 60Hz and have headroom for generational improvements. It'll probably require a little more focus on CPU power, removing cpu overhead (API), or maybe other bottlenecks that doubling framerate could cause (geometry processing and culling?).
Next-gen is going to be the first console where resolution has diminishing returns, where resolution can be varied as a smaller compromise than in previous generations. You can have resolution in ranges anywhere between 1080p and 4k dynamically with reconstruction. There's room for performance modes that sacrifice resolution or other visual quality to offer modes with higher frame rates. That was less true this gen, as 900p pixels are quite large on bigger tv screens. It was even more true in the PS360 gen, as there was a very stark difference between 720p and some of the games that were closer to SD.