But if they were actually as shit as you claim then the better players wouldn't be able to get the higher WR out of them either, which is obviously false.
And by flexibility I mean their ability to adapt to a changing battlefield. A T95 is inflexible because it min-maxes its stats for great firepower and durability with almost no mobility. A Pershing is flexible because it doesn't sacrifice a single stat but instead spreads around so it has good mobility and is very agile, while at the same time it has decent firepower which turns into great firepower if you play to its strenghts, its mobility and gun depression.
If a T95 picks a flank to go down it will stay on that flank for most of the match, being unable to help the team if the fighting shifts from that flank to another (ie, there were two tanks on that side and both are dead, you are now stuck trying to get to the other side of the map at 13 km/h). A pershing on the other hand can easily relocate to where its needed and fill a role that's needed. Wether the arty needs someone to light up the enemy tanks, a heavy needs someone to distract the tanks that pinned him down or a nasty TD has locked down an approach and needs a quick flanker to get dislodged.
Also, as far as brawling goes, having good alpha is not everything, neither is burst damage. Because brawls are protracted most of the time, the most important thing is either your armour or mobility and your DPM. During a brawl, you either need to have the toughness to weather the shots or the mobility to dance around the enemy and make him miss a good deal of his shots. Furthermore you need DPM because of said armour and dodging issues, because the last thing you want in a brawl is a 15 second reload time gun that has a good chance of missing our bouncing.
It's because of this that mediums excel (most of them atleast) at brawling over heavies, they are mobile enough to be able to dance around the enemy, and fire quickly enough to be able to minimize the effect of any stray or bounced shots on their DPM.