Years ago I was a barber working in a shop. Here's my observations about shape ups and hairlines or lack thereof)
Whether you'll get the Sherman Helmsley is half genetics, half your barber. If genetics say it's gonna go, there's nothing you can do about it. The other half is your barber. 9/10 if your barber has a heavy hand with the the shape-up clippers, that's when you run the risk of damage to the follicle and hair not growing back as strong as it once did. I can't count the times I had to create new hairlines for people b/c the last barber zeeked em. A sign of a good barber is you barely feel the clippers on your line (like a feather touch) and STILL have a crisp line.
Another way cats be catching L's is parents giving kids shape ups at a very young age. Kids follicles haven't fully developed so what sense does it really make to put the clippers (essentially a machine with a blade) all on the hairline? So asking the barber to give him the Cool Dr. Money shape-up does more damage than good. By the time he's 18 his line is straight up depleted.
I never understood barbers who pushed lines back. I ALWAYS talked to customers before I start the cut and if their natural line had issues I'd tell them straight up "yo, your line naturally grows this way. if you want it super straight I may have to take it back. Otherwise, I can shape it, keep it as low as possible, show you the mirror, and if you want it taken back more, let me know." And usually on the first pass I got it right.
Looks like Obama gets shape ups but probably just with scissors or a 'soft' technique with the clippers.