What has always intruged me since I became aware of said criteria, is allegedly how a difference of 5mm. of crank length, seat tilt, reach, handlebar angle etc., etc. can apparently make so much difference to riding comfort. Back in't day, we knew nothing of such refinements but still loved our bikes - !
Yes indeed we did, but you'll be amazed how many older riders I bike fit.
I recall fitting an eighty year old a couple of years ago, he was treating himself to a
Trek Domane as he was finding his Trek Madone a bit to harsh; understandable really. He has been perfectly happy with his 'bike fit' mind you, a position that he had enjoyed for years, yet to be thorough we checked it as it's is always a good idea to make sure the correct size bike is bought. Basing a new bike size using your current set up is valid but even more so if that current set up is the correct one. To reassure him I said what I always say, the correct bike fit is what you like, even if technicality it's wrong, providing you have no issues and you like it then that's perfect; we finish the fitting when you tell me that's where you want it.
I've just checked my records, in his case I raised the saddle 18mm, he tried it and instantly preferred it, 18mm is quite significant and it can effect what size bike is chosen. I can still recall him laughing and being cross at the same time, "years I've ridden like that Paul; I could have been an amazing rider
". Two years later he's still enjoying it, the last time I spoke to him he had
reprogrammed his di2, something many find to daunting to attempt; Chapeau I say!
Eighty years old, up for embracing change, embracing technology and treating himself to a new bike, I confess I find moments like that rather lovely and very life affirming; I hope I'm like that when I am eighty