That's true. The incoming FPS gamers will be critical about the gameplay as well.
And so what?
ED's own players craptalk the game all the time (just look at this depressing forum), and it still sold very well.
Do you actually believe some Doom player is going to buy ED expecting the shooting gameplay and combat mechanics to be as good as in Doom Eternal? Of course not.
In fact I doubt anyone is going to purchase and judge ED based only on on-foot action alone. Games with subpar gunplay like the Fallout series, Mass Effect and others sold millions of copies, the Fallout series sold even more than most first class FPS games out there, not because it had the best shooting mechanics surely, but because the sum of their parts is a lot better than if you just look at specific bits under a microscope.
Did you ever seen someone purchase GTA5 and then moaning than the airplane flying is so much worse than MS Flight Simulator, or that the car physics are so much worse than Assetto Corsa, or the ballistics are so less detailed than Sniper 3? And yet, GTA5 is one of the top sellers in gaming history, even though it's not the best at anything in particular.
ED is also not the best flying combat game, certainly not the best flight sim, nor the best trading game, not the best buggy driving game, and not the best exploration game. But the sum of it's parts results in a fantastic, unique game. Expanding it with on foot gameplay will turn it even more unique and special, make it's universe much richer, alive. It doesn't have to be the best fps game in the market not by a longshot to be successful.