Modern bikes are cheaper, more comfortable, safer, stronger, better made and more reliable than our 20 year old monsters. Doesn't make them nicer though...
They are not any more 'comfortable' nor safer, definitely not as strong as you would think and in no way more reliable - in fact I dont think any real progress has been made - whole runs of 2010 frames failed at their pivot points and had to be replaced. A whole run of 1997 frames cracked at their pivot points and had to be replaced. A whole run of 1990's bonded frames had their issues, a whole run of 2011 carbon frames had bonding issues. There are parallels throughout the decades. We're just focusing on our tiny point in life.
A £299 1990 bike with 21spd and cromo frame was considered a good starting point. A 2011 £399 bike with discs, front suspension and 24/7 spds is considered a good starting point - except, those forks are terrible, the BBs wear out way too quickly and the headsets had to be overtightened to stop the cups from moving in the frame.
The 1990 bike may have had STi that gummed up but everything else was fairly straight forward. Threaded headsets loosened and wore away the thread if not checked, BB's worked loose but could be tightened or serviced but there wasnt much else to worry about.
Modern bikes have too many BB standards. Many modern external BB's wear way too quickly when compared to an older internal BB. Older Shimano BB's wore too quickly but some models seemingly lasted forever
Modern disc brakes fail too quickly with their plastic pistons. Older bikes suffered from bad or lazy cantilever set-ups giving them a bad reputation. V-brakes were too snatchy, early disc brakes faded or were poorly designed
For every modern is better argument there is something to say that it isnt, for every older is better argument the same applies.
The rider's capabilities are almost forgotten these days. A competent rider could take just about any bike from any era and ride most of the trail centres (not including DH specific though I dont include that in any argument) without a worry. whereas some who only believe the hype think that its the bike or nothing.