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.
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.
That is really a bit of an oversimplification IMO. Please check, for example, the Netflix tech blog and read about what has changed in the past 10 years or so when it comes to architecting video processing and video delivery systems. There's a tremendous amount of engineering work there which advances the entire industry. For instance, it's not trivial to add live events to a VoD infrastructure at that scale — it's not like you just add a few more nodes and buy faster encoders.
