Fair enough. Certainly you can't throw just any challenge at a 1st level party, for example, and assume they can come up with a creative solution.
And the fact that I use modules only a small portion of the time may certainly play into things.
A module can easily be designed badly with no good solutions.
But, I don't consider the specific assumption that a L7+ party is going to be able to find a way to get off the ground unreasonable.
Like I said, that assumption wouldn't work in my group.
Savage Tide campaign - wizard had fly, but, that's only good for one person. At 7th level, it might be possible if the wizard rested, to cast enough fly spells to get everyone up, but, that adventure better have a nice built in break time.
Eberron Campaign - our party had no arcane caster and barely had a cleric. Not happening.
World's Largest Dungeon campaign - no arcane caster, no cleric - only favoured soul and later a bard. No fly.
Shackled City campaign - no arcane caster, no cleric. No fly. Healing was handled by a Binder.
Scarred Lands campaign - no arcane caster, did have a cleric. So, again, if there was resting time, the cleric might have been able to get two people up per day, maybe 3.
Now, I'm not saying my group was general at all. But, if you designed an adventure for my bunch assuming flight by 7th level, you'd be pretty much wrong. 15th? Sure, I'll buy that.