Regarding Aid: I think changing it to temp HP makes as much sense as anything, but I don't think it makes it better, just simpler. I've mostly seen people use it preemptively. After a long rest the cleric says "You 3 get 5 max hp, and you three get 10" and we go off. Often the change to max simply doesn't matter and is almost equivalent to temp hp, and temp hp is simpler to keep track of, perhaps. You've got to take a lot of damage and then get a lot of healing for a change in max HP to be noticeable vs thp. I've seen Aid used a bunch, but I think it actually only affected the battle 1 in 6 castings at best when used preemptively to raise max hp before the battle (ie, the player would have gone down if not for the aid boost). 5 thp is pretty meager, and the number of times this will be mostly a wasted spell slot when used will remain high. The biggest impact will be no longer waking up from unconscious, which actually I've not seen in practice, but can see how that would be used in some groups, in which case, this is a bit of a loss, though maybe that is intentional.
I guess I've never really been impressed by aid. I'm still not impressed, but will find it less annoying when the friendly cleric casts it on me as I only have to deal with thp, not changing my max. I guess that's improvement.