Respectfully - "It ruins the entire aspect of how forged alliance is intended to play" I don't believe anyone is the single authority on how FA is intended to play.
Turtling players are always at a disadvantage. This game is rock/paper/scissors. Look at what your opponent has and find their weakness then exploit it. There is always a counter. Always. A static base is literally a sitting target for you to pick apart. You can see exactly whats there and not.
As Zokora says some maps are strategy dependent. And some are not fun to play for that reason (imho). FAF has a bazillion fun maps, try a new one for a change.
T3 arty is easily defended. You get some shields and you reinforce them with a good number of engineers, making sure you have mass in the bank to spend on helping the shield regen.
The mass they spent on a T3 arty was not spent on units for them. So you should have a vastly larger army which should have wrecked your opponent. The time it takes to build a T3 arty is time you have to plan and execute a strategy.