I'd say 3 out of 6 of my bikes are keepers, to me. The 3 I'd give up are my off-topic commuter, a retro-mod I keep at the cabin and my 1960 Cinelli Model B. The commuter can be upgraded at any time, it probably has 30K hard miles on it. I'll ride it into the ground, get something else. The retro-mod is neat, but again, entirely replaceable. A couple years ago I would have said I wouldn't give up the Cinelli. But since then, I saw a Gran Sport equipped Maino that could replace the Cinelli. I still regret not buying it at the time (I'd sell the Cinelli to finance the deal) and it showed me I could upgrade the Cinelli.
What I don't think I could upgrade is a 1925 Frejus, a 1933 Frejus and an off-topic Cinelli. The 25 and 33 pretty much are irreplaceable. The 25 is a bit small, but I have no illusions of finding another. The off-topic Cinelli because it was the second production one made. The first is at the Cinelli factory. Also, it is a frame made from XCR tubing, which this year is the first steel tubing used in the TdF in the last 30 years. Good stuff.