> Green Card process has to be restarted from the square one
That's not fully true AFAIK. The green card (GC) is a 3-step process - PERM, I-140 and GC itself.
If you already have your I-140, unless your previous employer cancels your I-140 (which cost them a few thousand dollars), your new employer can just start over the third step of the process. This is not optimal, sure, but it is not square one either.