I have a few bikes, all Italian, some of which are the dream bikes while others are classics.
I find that the more desireable a bike is the better the chance of keeping it. Obviously, desireability will vary but usually means a classic sports bike. Classics refers to the likes of my 748 SP and F4, both stunning in design, function and desirability. Classics also refers to those bikes of the 1970s and '80s such as the 900 SS, 750 Sport, 750 GT, MHR and F1 Ducatis, Laverda SF in particular the SFC, Guzzis and of course the 4 cylinder MVs.
Also of importance is the engine capacity, 750 cc or larger. The smaller bikes are great but very limited in rideability (do you really want to do a four day trip on a 12 hp 125 cc bike from 1970 ... no)
With the modern bikes there seems to be a latest and greatest mentality which will often mean upgrading to the best spec bike every year or so. Great if you can afford the depreciation hit each time you change.
What is of interest is that the performance, in particular power, of the modern sports bikes is beyond what the average rider (at a guess 80% of us) can vaguely utilise on the race track let alone the road. I know quite a few riders on old dungas who would blitz modern sports bike riders except in a straight line. It's not hard to open the throttle to the max in a straight line. For me, the skill comes in riding around corners fast.
So far I have been in the fortunate position to buy a bike when I want to, without selling another to finance the purchase. I'm also fortunate to have bought a few bikes from the '70s and '80s which have appreciated in value or at least not depreciated. I'm also contemplating selling some of the bikes to acquire another relatively rare bike. The bikes to be sold would be a couple of 125 MVs which I know I can get anytime as they are readily available. The new bike is a classic Bimota from the early eighties. I've come across only two ( and one of those had been heavily modified) in the last year. I also expect it to appreciate in value should I go ahead and acquire it.
Anyhow, just my two bobs worth.