You literally just said mobile quarterbacks are changing the game and you need backups who are more like the starters.
You cannot tell me that whatever it is Justin Fields is doing is remotely close to Jackson, Allen, Tua, Hurts, Mahomes. I could go on about guys who still are pocket passers, because they are actual QBs, but you have to be cognizant of the run. Yeah, if you have Jackson or Allen, bringing in Chase Daniels to be your back up does you zero good. There are still guys Chase Daniel can backup and run their regular offense (poorly, as most backups do).