1) We don't know for sure that you can't increase caps somewhat through practice.
2) The temporary attribute boosts should add 5 to that figure.
3) Equipment and maybe shoes will likely still be able to increase attributes.
I wouldn't be surprised if you end up able to hit 80 or higher 3 point shooting as a balanced PF with max shooting, boosts and gear.
Also consider that the game weighs height differential on shot success against contest. At 6'10", you will be taller than 80% of the league meaning closeout success for opponents will be much lower. Even with lower attributes, I expect a stretch 4 to shoot a similar 3pt % as a guard, simply due to weaker contests.
Also, a quirk of the past versions of the game is that the cpu weighs your shooting ability in its defensive pressure. I had much more success with mid-70s three point shooters just because their man tended to sag off meaning I could take a jab-step to clear a little space and take a wide open three almost anytime. Once my ratings got above 80, the cpu defender tended to hug me at the three point line, so I had to work alot harder to get open.
All that said, if you're upset that low ratings caps mean you can't have a 7' Steph Curry, then you may be part of the problem.