If attacks always hit, I promice you, Champions will die far more often than the one-hits are doing now. I agree one-hits should be very rare, but always hitting/doing damage is going to leave dead or damage armies/champions even after the smallest of battles; which means constant healing; retreating; or reloading saved games after every single battle.
That's a big no. You're assuming they woudln't do anything to change how defense works. Since there wouldn't be anymore misses anymore the next thing defense would have to do is let you take and soak up bigger hits. You'd only die if you did something stupid like rush the enemy one unit at a time or something. In King's Bounty which has a combat system pretty close to this one, what you're scared of isn't an issue. In that game even the tiniest scratch on a unit kills people inside it. They're very vulnerable and can die out quickly forcing you to restock on more troops. But the thing is you can use strategy to avoid taking hits at all if you do things right. Soften them up with magic and ranged, let them come to you, send in decoys like summons to take the hits and surround them with your army. Battle ends, not a single casualty on your side but the enemy suffered in the 1000's. Doing that feels really good because it wasn't luck that made you get through unscathed but perfect timing and placement of your units. Which is how it should be.
As of now, there's still plenty of reloading saved games after battles because oh my wife just got oneshotted. Or my soverign or favorite leveled summon got complety crushed. The combat now more or less is don't get hit period at all, because if you do you die. If you do get hit even just once and happen to survive it'll most likely be a powerful blow that'll take you dozens of turns to regenerate the health back in a friendly city. The current way of things just isn't fun or time efficient at all.