If you made towers easier to take, those guilds would continue to dominate. It doesn't only work one way, it'd be harder for other guilds to hold the forts themselves. What makes you think the big 3 (Takaii doesn't count, there's always a guild like them, Rebellious, etc springing up) wouldn't continue to exert their player advantage and shift system to hold these towers longer than everyone else?

There is no solution other than to work harder for the forts yourself.