No no, that’s not my logic at all - don't get it twisted. That's your logic; I just flipped it into a different analogy to show the flaw (and yes, carrying 50 keys is silly, but you didn't argue they should be bulkier and heavier just because we are not mice - which was your logic to begin with).
On the contrary, my logic is that current devices are already heavy enough, and they should be lighter if possible. Changing a chassis material to achieve this is welcome, at least to me. Apple clearly agrees, otherwise we wouldn’t even be having this conversation; they wouldn’t be moving back to aluminum or introducing an Air model if they thought the current weight was just fine (assuming all this plays out of course).
At the end of the day, it’s good that there’s more choice in the market. If someone prefers the heavier, feature-packed Pro, great. If someone values a lighter, more comfortable device in-pocket, also great. The point is: weight does matter to some of us, and it’s worth designing for that too. For those that it doesn't matter, go grab what is best for you - but don't speak on behalf of everyone when you say this is useless and no one asked for it.