What made you think that? The stamina system would not stop you from swinging first. It wouldn't even stop you from slashing 2-3 times, but then you'd have to back off and catch your breath again, or your next swing might be slower than his. I don't think it would drop so fast that maulers would only need to block 3 strikes, because they are REALLY slow. For them, maybe after 5 blocks they could get faster than you, not before.
He doesn't need to be "faster then you" - just fast enough not to get spammed. The game allows you to counterattack with a much slower weapon. For instance, with a balanced 18/18 char, I can basically always counterattack with a GLB (which is really slow, in addition to being unbalanced and feeling slower then eg a long voulge which is slower on paper - don't understand why), a long voulge and other 80-ish speed weapons. However, with the long maul - which is 70ish speed - I can get spammed, and can't counterattack every time vs much faster weapons. Which is balanced, of course, since he also can't block me. (which answers the question why barmace isn't balanced - it can maintain a block-attack rythm vs everyone).
A few hits dropping your speed, and you can no longer attempt to "outspam", which leads to your inevitable death vs a weapon which isn't blockable. It's even worse when you consider a barmace or such which actually can maintain block-attack-block... and trying to spam even a str guy with a heirloomed (hence slower, but more likely to crush through block) barmace is very hard without a stamina bar.
I'm going to try and prove you wrong with my next build. In my opinion, it's possible to build a char with a long weapon that can spam non-shielders so they can't hit back. And against shielders, you equip a shieldbreaker with this build and need to block maybe 3 times, then their shield is gone and you can continue the spam. I think this is true because I have observed it many times while spectating other players when I was dead.
And from fighting ninjas with their masterwork katanas and 25+ agi builds I can tell you it's not true, and you can maintain block-attack-block vs them as well. Of course, sometimes they get you by chambering, but that isn't spam (unless, of course, it happened by luck). Actually I have less fear fighting a ninja then a normal balanced character, because I know I can survive about 2-3 katana hits, and they're dead in exactly one hit in the face with my masterwork glaive. Of course, you have to mentally prepare for the speed.
But if you can maintain block-attack-block vs someone with a masterwork katana and >25 agi, what build on earth are you planning? A 3 str long dagger one? I can counterattack vs nokeyboard who has I think a hafted blade and ~25-ish agi - except I have to be careful and do it immediately.
If someone blocks you and is immediately in range of his weapon (there's various footwork trickery you can do, of course), you have to block or you will get hit.
The "problem" of spam are people who don't block, or don't counterattack properly. Dead men don't swing.
(btw, of course, for sake of simplicity I'm ignoring chamber blocking, which isn't spam. I'm also ignoring various range tricks which look like spam but are not.)