There are only 2 old content pathfinders so far: WoD and Legion.
I think the first toon should have to do these on the ground. Then at max level, you can buy something that lets alts fly. That’s the rules we used to have pre-WoD. I see no reason to allow your first toon to fly.
As for pathfinder…
WoD is almost trivial. I did it in 2 afternoons on a level 100, except for the time-gated part. If you removed the time-gating, it’s fine as is. Maybe 2-4 hours work at level 120, and all your alts can fly in WoD.
Legion pathfinder is 100 times more time-consuming then WoD. And you don’t get it “just by playing”, now that Legion is not the latest. You cannot even START the Suramar campaign until you hit 110 (max level), then you have to do ALL of that, plus get Broken Shores reputation.
That is many hours of gameplay that normal players won’t do. They will play Legion while leveling from 100 to 110, then move on to BFA.