I've been thinking about this a lot recently. I tend to buy, fix, and flip 10-20 bikes a year and I've been beginning to wonder if I should hold on to some of these bikes as an investment rather than trying to make a quick buck.
We have a big cruiser cult following here in Denver and the newer bikes tend to at least hold their value. The limited edition or unique Felt's, Electra's, Dyno's, and Phats will all hold or gain value in 15-30 years. I have a feeling I'll regret selling the Felt Speedway (in for sale currently) as it was only made for 1 year, and will be collectible to both bike nuts, and Nascar/car racing fans. Items that appeals to more than 1 type of collector are always worth more from what I've seen. But whether that gain is greater than other investments is something different. A collectible bike today that was $550 new will likely be worth $2000+ plus in 30 years. But if you'd taken that $550 and was able to invest in in other things with an average 7% interest rate (assuming more gains than losses), that'd be $3912 in 30 years guaranteed.
But will I stop collecting or hoarding bikes... not likely. They're just too damn fun.