I'm saying you probably don't find them compelling because from your point of view, the problems don't look important to you. They don't from my point of view either. But my point of view is the wrong point of view. From their point of view this would be plenty to make me think twice and several times over past that from changing something so deeply fundamental to the system for what is a benefit that nobody who is actually paying the price for the package size seems to be particularly enthusiastic about. If the people paying the bandwidth bill aren't even that excited about a 5% reduction, then the cost/benefits analysis tips over into essentially "zero benefit, non-zero cost", and that's not very compelling.
Or you're not understanding how he meant it: there are countless ways to roll out such changes, a hard change is likely a very bad idea as you've correctly pointed out.
But it is possible to do it more gradually, I.e. by sneaking it in with a new API that's used by new npm version or similar.
But it was his choice to make, and it's fine that he didn't feel enough value in pursuing such a tiny file size change
