In general, I'm all for keeping an old bike if it fits well and rides well and then slowly upgrading components as they wear out/you want newer stuff. But if it's too small, that's a whole different ball of wax.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"