You show wisdom by not spending too much time trying to get your son to agree his homeowner choices are not logical or maybe not even sustainable without undue sacrifice.
You would probably be correct. but each married couple has to operate by their own rules and priorities.
Better just to enjoy the grandkids and not try to change or “correct” anything unless directly asked.
Perhaps the son’s family income is 4 times his, and now the son also has 2/3s less yard work.