My view on this is that turrets should remain relevant at any point in the game. Now, this is not to say to just buff them from the ground up, because tower diving needs to remain a thing. Buuuut, I think we can all agree it's dumb when 3 people dive you and you outplay them and they screw it all up yet in the end you still die and they all get away at like 10% hp. Also, to your point, why are they there really if they don't deter people from diving you? It may as well be another inhib.
So here's my idea. If under the current system being focused by a tower means you get hit with a little more damage on each hit until you lose aggro, here's what we do:
-Boost the bonus damage from successive hits just a little. Right now I believe the increase is 25% of the base damage up to 4, so at max rank it's doing double. Let's boost that up to 35%. So the first shot will do the same amount it does right now, the second one will do slightly more, you won't really notice the difference unless you are taking 3 or more shots.
- Increase the cap on the stacking damage to 5 stacks instead of 4, again this isn't a super big buff because you shouldn't really get there unless you misplay the dive
-The stacking damage will not be an effect of being continually targeted without losing aggro, instead it will be a debuff that lasts 10-15 seconds. So if you fail your dive you can't just walk out and try again 4 seconds later. Or repeatedly walk under tower taking bad trades.
-Finally, make every tier of tower have as much HP as outers do. I could be wrong about this but I believe towers have less hp as you move inwards, undo that shit. All of them have as much hp as outers.
Here's the point of all this, in a cerebral game like LOL there should almost never be a lead so big you can just turn your brain off and win. Even with a snowball going you need to be forced to play smart and correctly. Failed dives should be punishing at almost any point in the game barring a comically large lead.