It arguably isn’t anymore, although it looks like it is. Yes, you can buy prime frames and weapons, but can’t buy the mods or arcanes directly from the game to outfit them. Even then, prime weapons on their own are not the best in the game by a long shot, and a prime warframe does not guarantee it being “meta”.
The weapons that are the best are locked behind story quests, and even once you unlock one of those vendors, only one of them that you can buy with platinum is any good.
They were considering adding a destiny style story skip, but when they asked for feedback it was so negative they immediately relented.
you can buy prime frames and weapons, but can’t buy the mods or arcanes directly from the game to outfit them.
How is this not the opposite of a pay to win mechanic? Either you play the game to find the mod or find someone to buy the mod from.
An even cooler thing is operator progress, whereby no amount of paying can take you through.
The whole prime access is a p2w mechanic that prior to Liches was arguably considerably more important.
Now with Lich, Sister and Railjack weapons that are all play-only access.
Galvanized and Prime mods you can’t p2w.
Nevermind the whole Steel Path and incarnon adapters, where while a small set is plat available, and the only good weapon in that set is the Angstrum, and aforementioned Galvanized mods.
I don’t 100% agree with prime access being pay to win. It’s mostly pay to skip, you can use your platinum to buy pieces from other players for normally far less than the frame and weapons in access as well. I spent 5 bucks to get a protea prime from a clan mate knowing I wouldn’t have time to grind her and had tons of platinum (premium currency) from other trades with players. And I’m really bad at trading with players.