There are wayyyyy more characters who can backdoor like this than you thought. There’s even a certain support who can do so at level 1 but I’ll keep my secrets to myself.
At the current patch, there are 4 types of “doors” avaliable.
Front door: follow the minions route and attack from there (every character can do this)
Ledge door: attacking the sentry from the ledge since now you can easily jump up (before patch this was actually a skill only a few players know how. Sadly they patched it. Oh well)
Side door: going through the tunnel and shooting the sentry from there (for ranged characters mostly)
Backdoor: the hardest yet seemingly easiest to counter. It’s just the stupid teams that don’t see minimap (sentry lights up under attack) or the top bar (again sentry lights up under attack) OR the game telling you the sentry is under attack.
Honestly backdoor is perfectly balanced. Many a times I’ve killed the person backdooring before he has a chance to run. And even if he runs, I just communicate with my team to kill him when he runs to the center.
And as the guy above said, it reduces the team fights to 4v5. And if the 5 manages to push the minions to the sentry, minions will kill sentry much faster than any character can. The 5 can safely go back to base to kill the backdooring person and continue on rolling.