Possible solutions to towers being useless
You can only take up to your level -1 in tower shots in a period of 4s before the stacks fall off 3 at a time. You would die in 1 tower shot at level 1. 2 at 2. 3 at 3. Etc. Making aggro juggling still useful at times, but helps counter those dives where the one person who has the highest hp pool and defence just soaks shots or if a made or assassin is just being healed and shielded non stop while tower diving.
If you don't like that make turrets do 2% of your missing health while charging up over time like currently. I think it would cap at 6% missing hp if you used current method's. (so about 60 extra damage per shot with 1k hp missing). Doesn't seem like much but when you add up a total of 7 shots or more it starts to hurt and punishes squishies just as much as it does tanks for diving by making it harder to escape.
I mean we could always go the 3rd route and make them apply "Mortal" wounds instead of grievous. Making them essentially receive 50% less healing (from all sources) and shielding for 2s. TBH I thought of this one last and it's my favorite as it actually helps to counter squishy dives more than tank dives which are honestly the bigger problem.