They do have to keep drawing them as unicode assigns new codepoints. So they can't really be left alone, other than just leaving the old ones alone and only appending. But I would imagine this trend towards non-raster versions of emojis is more about making updates MUCH easier rather than "innovating emojis" (even if they claim that in their marketing slop)

So many of the newer code points are ZWJ patterns modifying existing emoji. If you already rigged the 3D shark emoji, when unicode decides that :shark: + ZWJ + :family of 3: has to resolve to :horrific shark attack involving a family of 3:, at least that's not too hard.