lol and axis get the best AT weapons and armor that have more bang for their buck plus they are easy to use and easy to play. not allies they can be frustrating as fuck as well provide a huge mental migraine strain with all that micro...
If you have ever played Axis before, in automatch, you would know how wrong parts of your statement are. Late game is easier for Axis than Allies sure, but as much trouble as you are having late game with Allies, it is the same for Axis players early and mid game. Considering USF have the best mainline infantry with the most utility, it really sucks having weak popgun Volks and expensive to reinforce Grens that wipe to any explosive sneeze. I have played both sides and found both have their frustrating times of the game. If you are really having that much trouble you can post replays and ask for help in the Strategies section.
As for your OP, Shermans really don't need an upgun upgrade, they got a Pen buff in the last patch and are excellent AI tanks with decent AT utility. An upgrade on them could also lead to spamming them with the upgrade over Jacksons and making them too similar to E8s.