> hosting video at scale,

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.

Without having access to the source code we can only speculate but I believe even in those days YouTube already outgrew vertical scaling and thus had to be built as a horizontally-scalable system. That is the hard part.

Adding extra nodes to an existing horizontally-scalable system (that has already been operating and has its bugs ironed out) is much easier.