Like, did they even think about cost when designing this the first time?
More than likely, Prime Video making their numbers look better makes AWS' numbers look (slightly) worse, because they're doing a little less business. In the overarching grand scheme of things, this will save Amazon some amount of physical computing resources they weren't getting paid by an outside customer for, but good luck figuring out how much that actual real world savings is.
Obviously no, only after managers complained.
The latter of course helping Amazon market "serverless" to the unwashed masses as a "solution".