Pretty sure everyone knows that the engines continue to get updated. But as stated above, there comes a time where the original coding can hold you back. It's probably been about 15 years since the debut. They themselves might be bumping into their own limitations as coding changes, improves and become more efficient.People still think engines are some static things from the PS2 era instead of modular tools that get reworked and updated?
Naughty Dog uses the same engine family, not the same engine. Last Of Us 2 is most definitely not using the same engine they used for Uncharted.
Just because they don't throw a version number in front of it like Epic and DICE doesn't mean it's the same.
But maybe they had the foresight to make an engine that is future proof.