I do agree that swimming does require less super-fine mastery of movements, but still, in violin, you don’t develop that super-fine dexterity until you’re in your teens/adulthood. This still requires that you started early enough to build the foundation as a hatchling - you can’t even approach that dexterity without that base.
Still, I do agree with you that in swimming, the need for super-fine technique is reduced, and thus there’s more of a chance for a late swimmer to gain good technique. Actually, I’d be surprised if technique alone accounts for more than a second per 100 at the elite levels, since everyone there has essentially solid technique.
I also think though that the ENGINE component makes the technique less critical in swimming than violin, which clearly has no engine component. If you’ve got the biggest engine in the world, you could probably get away with 98% perfect form. Come to think of it, there are some who would categorize Janet Evans and her windmill stroke as such back in the day.
I disagree though with your notion that Phelps could take up swimming at age 21 and even reach the Olympics. He would be too far along in age at that point to have access to all the specialized coaches, pool time, and resources that were required for his entire teen life to get to where he is today. I also doubt he’d be able to train as much/hard as he does now if he started so late. It would also be much, much more difficult for him to pick up swim technique as a 21 year old with no pool experience, than before. I think it’d be a stretch to say Phelps with zero pool experience walking into the sport at age 21 would win gold at the Olympics. The logistics of the resources wouldn’t work out, and the technique/training limitations are also massive. He’d clearly be a swim monster, and might even be 99% as fast as he is now, but that 1% is a HUGE gap in world of elite swimming - like the difference between 1st place and not even making the team.
Well, this is all conjecture anyway but I will point out actually 1% is not quite as great as you might think. For example, the WR for 200 LCM is 1:42.00 which is conveniently 102 sec and MP’s AR is 1:42.96 or about 103 sec; so if he went 1% slower that would be about 1:44, and Sun Yang and Tae Won Park tied for the silver in 1:44.93. Of course, both the AR and WR were in the tech suits but you get my point.
Anyway, you have a point about the access to pool time, coaches, etc. Bob Bowman prob wouldn’t take a 21 yr old Phelps under his wing the same way he did with the 11 yr old Phelps. In truth I was exaggerating a bit to make my original point:)