I think this could be interesting. Personally, I have nothing against trying to make defensive play a bit more viable than it is currently, as long as it's done well -- sure, it doesn't win games right now, but that doesn't answer the question of if it should be able to win games. Suggestions should be aimed around what we want to change the game to encourage, not just how the game is now -- if we make defense stronger and it's still useless, well, who cares, that doesn't change anything. The question is, do we want to try and make defense not-useless? However, I would be more concerned about this idea's offensive applications. Let's say my engineers build a bunch of turrets in a few key areas, near frontlines but just out of range of them. Then I upgrade, and suddenly an entire row of turrets is able to hit the enemy at once. Is that a good thing or a bad thing?
Turtling fails because tech/res flow is more important than tickets. If you turtle to deplete their tickets, they will generally get heavies/arty way before you do and have 10+ tanks dominating your team.