I don't profess to know the minutiae of the lodging business, but, assuming there are still enough rooms for potential elites to get their early check-ins, I can't see why a hotel wouldn't give an early arrival a room if it's ready. The incremental cost in electricity and water is probably offset by the incremental savings in electricity and water from people who check out early. I'll never demand to be let in early, but I think it's petty to charge for an early check-in if a room is ready, especially if I've already claimed a specific room as mine (such as by checking in on an app).

I do think most check-in (and check-out) times would be better revised a couple of hours earlier. It's more often it would help me to check in before noon (get settled in and then go for lunch) than stay in a room until noon.

Budget hotels, in my experience, are more willing to let in an early arrival.

The earliest I've ever checked in was 4:55am, at the Arlington hotel in Dublin, where the receptionist offered a room proactively without extra charge even though it was about 10 hours before the check-in time.

Seth