Well the truth is that if you can do your own wrenching, you never really have to buy a new saw. You can get a hurtin' saw or a dead saw and bring her back to life with minimal cost. You guys are right about the 51/55 chassis being reliable firewood producers. Not as sought after on boards like this where the latest/greatest will always dominate the discussion. But any saw that will reliably fill the truck with firewood hits the bulls eye in it's own way.
And saws that are generation or 2 back like the 55's and the 272/670 chassis are easier to find for projects. The older the saw, the less likely folks will want to spend money on them. Especially on saws in the 50cc class that can be replaced for the cost of a shop repair. So they're more likely to buy themselves a new saw and leave behind a nice project for the rest of us.