1: Counterplay, melee need to be in melee to hit the tower. Demolition needs the tank to sit near it for several seconds. This is significantly easier to reach/threaten than the tank maneuvering to that position to vanguard and the adc 600 units back being the DPS on the turret.
2a: A niche, both for items and melee in general. Sheen items (especially trinity) get to shine better in their role as tower-shredders.
2b: As for melees, my standpoint is "Let Melee Have Nice Things". By and large, the short end of the stick goes to melee, because having range is such an advantage.
3: Adcs will still be good at the job. Even with this specific rollout of changes, it's likely that ADCs will still be the preferred wall-breakers simply because of their relative safety, and the fact that they're better in team fights. Additionally, they're allowed to build full-glass which means the total dps they can bring to bear on a tower might still be higher.
To answer your specific reasoning, we're past the days where "melee comes online quicker" is something I can subscribe to. Sure, technically they do, but ADCs come online fast enough (and towers are frail enough) for that to not be exceptionally relevant.
Additionally, there has nearly always been a marksman on each team in every serious game of league, and I don't foresee that ever changing. They're the best scalers, the best tower pushers, and they do more than that (because if they literally only did those two things there'd be like, 5 marksmen at the most) (ex: Utility/Lane bullying/Aoe Ult/Waveclear&Siege).
Thus, I figure marksmen can take the power hit, and this is one of the few ways that's easy to implement/measure, only hits their tower clear and not their teamfight/survivability, and hits the whole class evenly without playing whack-a-mole with the FotM ADC. High on game health, low on relative player dissatisfaction.
Wow, that got wall-of-texty, sorry to whoever had to wade through that (Also, one more change I'd like to make would be removing that +10% minion. damage they randomly threw in as compensation for 300 more HP on turrets, 100% unnecessary).
tldr: "Let Melee Have Nice Things", adcs will still probably be better at tower-smashing even with those changes.