> It might be reasonable to go without a garbage collector if your whole lifetime is so short.

That isn't how lambda works. A single lambda instance can run for 10s of minutes handling thousands or requests. So not cleaning up memory sounds like a problem to me since you're billed by the GB/second

If you throw away your memory on every request, every request is a cold start. You've reduced your one-time startup cost but now you pay that reduced cost on every request.