The new features are an order of magnitude less complexity to build than the main feature - hosting video at scale, which is complete and just requires maintenance.
I'm going on a limb here and saying that the scale that YouTube was running on back in 2010-2015 is not the same scale as now, and if they had left their whole infrastructure unchanged, a "finished product", so to speak, the site would have been feeling dated and would have eventually been killed off.
Adding extra nodes to an existing horizontally-scalable system (that has already been operating and has its bugs ironed out) is much easier.