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.