I would suggest not assuming - go try/ride a new bike. It will be obvious how much better new is, compared to 'old school'.
The difference in handling from an old 26 to the newer 27.5 and 29 is remarkable. The function of drivetrain, suspension, brakes, wheels, everything is better Seriously...
Your concern for 'wheelbase' is unfounded. The new geometry of the recent generation is a HUGE improvement over anything old school. New offers not only better handling, but a larger margin of safety and comfort.
I already had a top line old school bike; and with some easy upgrade, I get some of the 'effect' of new, but not all and certainly not for all situations.
But I'm quite certain that a 'new' bike will be in the cards for me, in the near future, as my riding time and skills return and improve. All still keeping my riding at a conservative level.
Buying 'old' and thinking to 'upgrade', I believe is foolish, and way more spendy than getting a new bike now. Not considering on the needed 'kludge' to get the upgrades to even fit.
Go, ride a new bike; then ride something old school - the difference will be more than obvious.
Rode On
Yuri