Fighting against multiple oponents will become a lot harder. But fighting one enemy will be easier as well.
In cRPG being shielder means you're good at fighting multiple enemies. But that doesn't make sense, I mean fighting multiple enemies doesn't make sense at all. I'm pretty sure shielders weren't meant to aggro five or more enemy fighters, then either backpedal or spin, that way forcing the enemy to teamkill themselves.
Makes perfect sense in cRPG because it was always like that (not just for shielders) but it's quite silly actually.
It's not silly it's a core basic for Warband class balance.
2v1 vs non-shielder: If the attackers have half a brain they attack at same time with different directional swings, single guy can only parry one direction. He starts taking hits.
2v1 vs shielder: Directional attacks don't matter. Shielder can block any direction and is able to focus more counter attacking than desperate parries.
In exchange for this advantage vs multiple opponents (plus protection from ranged), 1h weps are weaker and shielders get a speed penalty from their extra gear. You are proposing to obliterate this aspect of shielding and just turn them into duelists, completely & radically change the class.
Why? I dunno? Cuz you dislike them personally even though you have never played one seriously. Terrible. Did you get beat by a shielder and make this as a ragepost?
If there's a problem with shielders who "turtle", aka are super defensive and have nearly indestructable shields, then just cap shield skill at 5 and maybe nerf some of the 2 slot shields. A lot easier than radically changing a class. People who do that are just newbies anyways. From my observations on NA at at least, basically none of the best shielders use 2 slot shields, and none have more than 5 shield skill, and all of them play very offensively and walk away victorious from 1v2 not by "turtling" for minutes but by aggressively timing their attacks into openings.