The scaling on many damage skills is lowered, on top of a nerf to all battlerage skill cooldowns via battlefocus
Melee can hit 50% parry unbuffed (80% when battle focus is up) with just 1500 strength (actually not that much in 4.5)
Tanks can hit 15% parry, 20% evasion, and 28% block with a minimal gear setup in 4.5 (in actuality it's 43% block if you run defense. Under the current way mitigations roll, this would mean you'd have 78% frontal mitigation
Having mitigations (evasion, parry, shield block) in 4.5 not only block full damage, but roll additively, as well as prevent CC would be completely unbalanced.
How would this be unbalanced? A tank's literal job is to stall and survive. They really cannot doll out much damage. The entire point of rolling into these stats is to be evasive and to have a high amount of sustain. This includes nullifying damage received, avoiding damage altogether and being the frontline for multiple individuals. 4.5 completely trashes the ability to be a tank, and more to the point, completely deletes the desire to be one due to the shit hand tanks are dealt.
Some of those avoidance numbers you stated aren't adding up off the cuff, but I'm not in game to verify and I don't remember all the numbers. Sounds semi legit with some hand gems and a cape, but consider this:
As an additive set of statistics, you'd be far better equipped as a tank to deal with say, archers, who cannot be parried unless you are dual wielding with a passive from battlerage. Using the numbers you provided, you'd be able to avoid ~48% of the shots an archer takes at you (we all know this game fails beneficial rolls as often as it can though).
As an individually calculated set of statistics, you'd be near worthless as a tank vs archers, with a 20% chance and a 28% chance respectively to avoid a shot from them. Archers have the highest attack rate of all classes in the game, and with the ability to stack pdef pen they can hurt pretty hard. You'd get melted. Especially considering this game already has an extreme tendency to roll on the side of failure, even with statistically high chances of success.
And for both of the cases above, lets not forget the role that Focus plays into mitigating avoidance. In 4.5 focus becomes even more powerful than it already is, which is some real bullshit.
Also considering the new abilities the archery tree gets in 4.5, they're going to be even more of a bane to tanky types.
The additive avoidance skillset is a far superior method to individually calculated ones.
Meanwhile, with the meta changing, DPS melee can, as you said, easily hit 50% parry. How is it more fair to allow a DPS style class fuck with a tank to the point where it can effectively avoid 50% damage from that tank, while whittling the tank down, where as with the way the 4.5 meta will work it is extremely statistically unlikely that the tank will be able to ward off that DPS for long? Also keeping in mind the severe amount of CC melee classes can have to keep pretty much any class locked into the dirt, that is the unbalanced part.
Also also, battlerage was the
least nerfed of all the skilltrees as a whole. Some of the biggest hitting skills in battlerage got even more powerful (whirlwind slash up to 300% damage what the actual shit), precision strike got zero meaningful nerfs and still keeps a ridiculous damage scale, and now battlerage users get to walk around with two severe damage boosts because frenzy got a double physical attack boost and no longer stuns. What the fuck?
The *only* meaningful nerf was to behind enemy lines which finally isn't dumb stupid powerful and CC breaking.
Overall with the meta heading the way it is, it is going to destroy several viable playstyles and it only serves to pave the way for melee DPS more than it already does.