Your higher tier units are already more badass then lower ones in the current game. Realistic mode just further push it to exaggerated levels. The moment your opponent faction start losing they come back with their army compromised mostly of recruits from this point you can just f1 f3 a couple of tier 5 units, alt tab, watch youtube, tab back in with an ensured victory with 0 unit losses. If that sort of game play is enjoyable for you there's already campaign options to reduce damage taken from your units.
I still think higher health values for higher tier units preferable to complete invincibility. Also a minor nerf to projectile damage.
Another point for different health value would be giving a reason to actually spend some focus points in combat skills because currently you can already 1-hit most units as a player.
I find it weird how the idea of losing units is inacceptable when there so easily replaceable plus we've received a new way to quickly gain experience from discarding items. I know it is a single player and it's alright to enjoy having an easier time but like I said, the options are already there in the menu.