Yeah, I've heard the same thing, but there are plenty examples on this site and elsewhere of successful winter squash landraces. Just not many of summer squash. I've read Carol Deppe's The Resilient Gardener, but will add her book on plant breeding to my reading list.
I'm thinking that the offspring of two good summer squash should mostly be good summer squash, as long as I select against traits that make the plants inedible or unproductive in the future, those traits shouldn't just develop in the first few generations. My thinking may be wrong on this, but I haven't seen examples of squash reverting to a wild form without wild pollen being introduced somewhere.
Thanks for the reply and advice.