Yeah, its a funny one.
I originally got interested in old bikes because they were all I could afford at the time.
Since I began racing VMX, I reckon I've spent as much on old bikes as I would have on buying a new two-stroke, racing it regularly and replacing it every year...
But I definitely don't regret the money I've spent, and the old bikes still float my boat for a number of reasons:
I love taking a "worthless pile of junk" and making it into something can be enjoyed;
I love rebuilding/preserving things that would otherwise be thrown away;
I love being able to ride something vaguely near its limit (unlike a decent modern, where I ride around at one-third throttle);
I love community that comes with old bikes - people are keen to help each other out and I am yet to meet a genuine dickhead through old bikes;
I love knowing that the old bikes aren't depreciating if they sit in the shed and are ignored for months at a time (even though I'm not into the "this here classic is appreciating by the day" way of thinking);
I love the lack of egos and bullsh at old bike events (at least compared to modern events);
I love discovering the old bikes that work beautifully as a bike, (even when they don't do anything particularly well by modern standards);
AFAIK, there's no modern equivilant to CD or HBBB.
None of this is meant to perpetuate the "Maico 490 [or whatever] was the best bike ever built" line - if they were that good, they wouldn't have stopped making them... New bikes do their thing very well, there's no doubt about it.
I've got an '00 model that eats my similar '94 model bike, which eats my similar '89 model - even for a gumby like me, the engineers have earned their pay since everything began to fit the same mould in the late 1980s.