Within the relatively short space in which people are focussed on progressing techniques on this hardware. Software techniques continue to evolve and these are often key to extracting performance from old hardware, which is why we keep seeing
new games or ports released on very old hardware, delivering what would have been considered impossible during the hardware's original lifetime.
How much more could've been extracted from PS4 and Xbox One had the current gen consoles not released?
Right, but you you know what these numbers mean in real terms of what games can deliver changes over the course of a console's generation because the equivalent of hardware drivers and the APIs improve over time. This improvement cycle is continuous and only stops when effort is diverted to the successor platform.
If gameplay and graphics of the level of The Last of Us Part II could have been delivered at the scary of the PS4's generation, it would have been. But it wasn't. That was only deliverable over time, both with experience and development of improved software techniques and improvements of the APIs and development tools.