The close-quarters scenario for instance: if an opponent with a shorter, faster weapons gets too close you may be at a disadvantage, yes. However: considering that you have the superior reach, wasn't it your fault that the opponent got too close in the first place? You always want to fight at the range your weapon is best at. If you end up fighting at the opponent's preferred range, then that's your fault and not your weapon's.
Well, yes. If you play it perfectly, the enemy with a shorter weapon will never touch you. But under the same assumption, an enemy with an even longer weapon like a lance or a pike will always defeat you, as you could never touch him, either.
To be fair, if you never made a mistake, you´d win most battles regardless of weapons involved.
Regarding the fact that you can't use a shield with a polearm, that's not a problem if you are wearing full plate.
If possible, always wear plate armor to battle, as it simply is the best protection there is. But it has all of its own restrictions; it is expensive, needs to be tailor-made to properly fit, needs maintenence and often a second person to properly put on.
In battle field context, I agree that plate+polearm is extremely viable. That is why it was such a popular combination in the late middle ages, plate armor and a pollaxe, for example, which gives you all kinds of options to kill other armored opponents (smash, grab, hook, lever...) with yourself being well protected.
However, for example, if your opponents dont wear much armor, a lighter and faster weapon like a sword would be just as deadly while being less exhausting. Or if you are up against fast and evasive enemies who shoot and retreat, you can either try to catch up until weight and heat build up slowly drain all your stamina or a lucky arrow finally finds a weak spot in your armor. If you need to kill them now (no tactical retreat possible), probably ditch some weight and use a shield instead to to have a chance of catching up. Muddy or unfavorable terrain in general can also have a huge impact on how useful certain weapon and armor combinations are...
It is always a combination of what you have and do, what your enemy has and does and various other circumstances which determine how effective something is in a given context. There are some things more likely to be viable more of the time and some which are only good in some rare cases... but there simply is nothing that will
always be the best (in real life, that is. Given that this is a game, the devs
could of course programm something that is the flat out best choice ever; im just saying if they stick to a somewhat realistical approach, that wont be a problem).