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.

Critical mistake thinking paywall is pay to win. There is a difference, one is locked, the other can open their wallets but don't have to, but can. Any type of advantage that can be bought with money IS pay to win. The key word is advantage and can be applied to any view point, like 'skipping' and 'easier'. A person that pays will always be ahead of a person that doesn't, hence paying to win.

I mean, if you are trying to argue against this means you probably wouldn't mind buying premium bullets in a video game, like a CEO, John Riccitiello, suggested. Premium bullets would be very convenient and make killing players easier! You wouldn't need to grind levels as long with those boosts!

Rule of thumb is to ask: would paying allow me to finish the game faster? Yes means pay to win.