Sounds like you're making 2 assumptions here:
1) You're only going to be using it on a 747 (What are you going to do if you get to the airport and there's been an equipment change...not easy to change down from a 747 but I'm sure it's been done before....and the one rule of air travel is NEVER say "never"!)
2) That you'll never get on board and have the FA tell you "Sorry but the closet is full"...seen it happen.

I just think you're making some pretty big assumptions that a FA would not have a problem with letting you bring a 26" bag on board. I've seen them refuse bags that size. You're also assuming that security will let you through with a bag that size...and yes I've seen them turn pax away at ORD with "too big" bags.

I suppose you can try it if you want...but you've been forewarned about the possibility of FAs making you gate-check the bag, and also the reactions of your fellow pax (probably a harsher reaction than the FAs....and do you really want to sit surrounded by ticked-off pax for a transcon or trans-ATL or trans-PAC flight?) JMHO...