I see your point, but I think I don't agree with it because I do not have the same reaction in front of an overprotected shield user. If he just keep backwalking with his shield up, most of the time I'll let him go away and won't go berserk on his shield 'till it breaks. If he wanna fight and come back, I rape him =)
I agree those situations are kinda boring but I don't think the stamina thing would make it worst. But, against other good players, I think It could make some fights more "strategical", trying to make him waste his strenght while you're trying to keep yours up.
But there's one sure thing : the stamina implementation wouldn't work with the actual kick because of what you explained and the difficulty to have an opening against those guys.
OR it would work if they make it so when you just keep your shield up and let it being hit you loose more stamina every new hit** as it would be for real : you can take 1 hit on your shield easily, a second one, maybe a third one if you're well trained, but every time you'll get another hit it would hurt your arm, weaken you, and maybe makes you fall at a certain point.
This situation would force the guy hiding behind his shield to react against his ennemy's attacks if he don't wanna get overwhelmed by them.
And it would adjust the not-so-balanced shield mechanic.
** : like the first hit would make you loose 1 stamina points, the second 2, then 4, 8, 14, 22, 32 etc... with your weapon's effect added. Or something like this.