I'd go for the "if you regularly ride it, keep it - if not, sell it" rule of thumb.
At the minute I have two bikes that I regularly ride (in Greece, a '95 Kilauea and, on the Isle of Man, a Singular Hummingbird singlespeed).
So I have my 1988 Explosif and my blue '95 Explosif SS that I really should get rid of.
However, whether I do or not doesn't make much difference, as neither of them is worth very much, if anything (about what you'd spend on a week or two's shopping probably) plus I don't tend to spend much money on bikes, except for consumables like tyres, chains and brake pads etc.
In fact I probably spend more on riding gear (shorts, jerseys, shoes, gloves and stuff) than on the bikes themselves, so that wouldn't change even if I only had one bike, nor is storage space an issue.
So, unless someone walks in one day and makes me an offer I can't refuse, I'll just keep them (and I've had the '88 Explosif for 21 years anyway).
Generally speaking though, for me nowadays, it's more about the riding than the bike - as long as I have a bike to ride that I feel really suits the riding that I like to do (I have) and it is well maintained and mechanically as perfect as I can make it (it is) then I'd rather be out riding it than fettling it in my workshop.
It's like I enjoy wheel building, but I only build the wheels that I need, not for a bike that I never ride......