Any adventure that depends entirely on an impassible ravine is a pretty weak adventure. So what if birdman can follow him back to the lair? As you note, this isnt anything a 3rd level spell cant do. Or a 2nd level spell (levitate) used creatively (build kite wings to propel yourself). Birdman is still on the other side of the ravine from his buddies so at greater risk from Guy McCutsbridge, so is probably going to be stomped unless Guy forgot to pack a bow, in which case, shame on him. If the DM wants Guy McCutsbridge to escape so bad, he shouldn't have designed him like a chump that cant take one member of the party on mano y birdo. I'll also note that Birdman is also down an extra death save should he get reduced to 0HP, as he's gonna take falling damage. Watch your cloaca Birdman!
One man flight lets you avoid some problems that aren't really interesting after low level anyways. Telling different stories is part of why I like being a DM. I don't keep my PC's dirt farmers for 15 levels so I can throw the same pit traps and rivers at them or make them count rations. I don't sweat the small stuff in designing challenges. If I run a campaign and allow Aarakocra, it just shifts the bar of what generally constitutes small stuff to earlier in the level cycle.
I like that 5E just said "Yup, Birdman can fly" as opposed to some kind of stupid chart like they would have had in 3rd edition that grants Flap at level X, then Flutter at level Y, then finally Fly at level Z. I'm hoping when half-giants come out for Darksun they just straight up say "Yup, half-giants are large! Enjoy your +5 strength and smashing stuff!" instead of pretending 7 feet tall is big enough as they would have done in 3E/4E.
I also find it ridiculous to insist that a race of creatures designed specifically to attack from the air loses that ability the moment they become a PC. Ban it if you want, but it seems petty to neuter it by some arbitrary ruling that you can't fly and attack. Manticores also don't have hover and are you going to argue they cant shoot spikes while flying? Hover just means you don't fall when reduced to 0 speed because you're effectively levitating.