This is a terrible decision, it's just pushing players to create builds for no reason.
Getting rid of invulnerability means: You can't have warframes who can get easily killed in high level missions, go to a high level mission.
For example, when we enter into high level missions... when our shield is down, enemies can easily kill the warframe in one shot...
What we used to do is: enter into void mode, get the warframe invulnerability, and then wait till the shield goes up again.
Now: we used the shield booster to get the shield up immediately so we don't get killed...Or we use rolling guard... Notice anything?
YES... you force the players to use new build for no reason... And people just going to use warframes who are good at surviving instead of those warframe who are not 😞 Because less people use warframes who are bad at surviving, you WILL change those warframes who are good at surviving AGAIN..
See where this is going?
One day, when they get tired...players are going to ask themselves: seriously is this even worth my time to invest? I put all my time on one focus ability, on one weapon...and it's getting nerfed.. People who don't play those will think: HA I don't play it anyway~~ Why did you guys even invest on it?~~
And that's the part I am really really worried about. You need to reward those who spend lot of time on building the skills, not create any conflicts between the players, don't let the players feel tired to hear about your ideas.