From what I hear from players of the game, there's no actual gameplay advantages locked behind a paywall, only cosmetic items.

Pretty much. There's lots of content that can be reached much more quickly by paying, but I've never felt any drawback taking the long way around. It's great fun without paying a cent.

It's a very good game that makes me feel good the entire time while playing it the long slow way. When I've occasionally tossed the developers some money as appreciation, my purchase generally results in some random later mechanic getting introduced into my game sooner. It's amusing, but not particularly noticable, since everyone in my friend group has generally just unlocked something interesting, at any given moment, without paying.

Warframe's free play feels so generous, in stark contrast to World of Warcraft where paid play felt stingy.

And other people buying their way to the end doesn't affect my experience at all. They just look cool in the lobby.