That would probably work great for a kid who will be happy in their seat for 2.5 hours (the length of our flight). Our 17mo would *not* be happy, unless she managed to fall asleep. In a car, we could (and do) stop and pull her out to let her have a different position, etc. If I do that on a flight, there are 2 consequences I'm not happy with:
1. It will make her think it's ok to get out in a moving vehicle.
2. It negates the whole point of having her in her carseat to begin with.
Otoh, if I put her in my wrap (literally tying her to me) anytime we're asked to strap in and anytime she's sleeping, we won't disturb *everyone* else because she's screaming after the first hour and we won't be sacrificing very much safety, imo.
That said, this is our first flight. If we ever fly again, she'll already be over the 2yo age cutoff, and it won't matter anyway.