What you suggest actually can't work, it means applications have no control themselves what engine they run on. Browser engines can't just be a config setting, you're basically asking for chaos.
Also you should review what "ad hominem" means. Saying "no one intellectually honest would say 2 + 2 is 5" is not ad hominem.
Also you should review what "ad hominem" means. Saying "no one intellectually honest would say 2 + 2 is 5" is not ad hominem.