Is this change to the Protection fighting style balanced?

I feel the Protection fighting style has some problems that make it mechanically marginal at best, and not very much fun to play with. In brief, while it’s a decent defense bonus, your chance of actually being useful is pretty low. There’s around a 20% chance, each time you use Protection, that you’ll do any good — that is to say, the attacker’s initial roll would have hit, but their disadvantage roll has failed to hit. Add to that the positioning requirement and the cost of your reaction, and it often feels like a waste to use this ability.

I’ve been considering a few ways to beef up the Protection style to make it feel better without making it completely overpowered, and I want to get some feedback.

My proposal is instead of giving the disadvantage against just the one attack, make Protection act a bit like the Shield spell — it’s a reaction to use it when an ally is attacked, and then any attacks on that ally have disadvantage until your next turn.

What do you think? Is this too powerful?