Given that those talented engineers were not trained de novo by any of those companies it is a fair bet that they joined for a potential upside and could be bought by Lyft+GM under the right circumstances. There were people willing to actually go work for Uber in this space so, obviously, anything is possible...

This can change imo, lyft still hasn't got a bad reputation (I live in SF and have never heard neither good or bad things about being an engineer there).

Engineer culture is complex and moving, and I believe just a few very good/bad decisions can tip the scale either way.

Who the heck wants to enter the GM tech culture?