personally i think your coach is a bit out of order.
there are a couple of things that are correct but to suggest that you should never ride the bike again?
early 90's mountain bikes went very race orientated, previous to that they were quite upright with riser bars etc but when mtb's became popular the racing took over so the geometry changed into an almost roadie set up as speed and performance was paramount.
as has been said above the geometry then changed again as suspension became popular the front ends got higher and this meant a change in head angles/stem lengths to compensate.
also don't forget that in the early 90's trail centres etc didn't exist, courses have got deliberately harder because modern full suspension bikes can cope with rocks and drops that early bikes were simply not designed for. back in the day i was riding off road but by comparison with the off road i now regularly ride in wales and the alps is so different.
to sum up, if you're riding the same places as you were 20 years ago you should be fine on the bike, if on the other hand you want to hit a rocky trail centre in the middle of wales then i'd suggest something a little more suited.
personally i have loads of bikes, the retro stuff gets used alot around the new forest, and it's actually perfect for these trails.
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