You could.... COULD make something like this work in game but there would have to be so many restrictions and so many down points to make the planet busting weapons.. effectively useless. Ideas i'd have to make such a thing balanced.
Cooldown timer: like you could fire it but only once a month or something crazy like that, Firing Timer: How long it takes to actually fire the dam thing Could make for some interesting fast responding defensive measures
Costs of construction: Heavy Real heavy,
Controls: Could make it so that to even Man such a weapon takes a certain amount of people that way no one person could ever do it.
Impact Damage: The Damage would have to be Limited to destroying - the Surface- And No actually Obliterating the planet. I doubt planets will respawn
Shielding: Such as all things Weapons need shields against them.. And making shielding against a planet buster would have to be more easy to make then the weapon.
Zones: Have the Weapon only work in heavy PVP zones, So specific areas only Can this weapon Fire or even exist in.
Power: How much power the Weapon needs. Example have it so it requires all the ships power to fire meaning said ship is Defenseless for counter attack.
Type of weapon: You could easily have Different type of weapons.. Like Lasers, Ion Cannons, Projectiles, Plasma etc kind of weapons that all have different effects when fired at a planet destroying level. Speed: Any ship or massive structure that has this Planet destroying weapon cap its speed so it can't just fly around the universe too fast, Give Defenders a chance to detect this SOB that is coming to end them. Sensors: Make it impossibly hard to hide such a weapon... so if a weapon is built with a certain power level capable of destroying worlds have it show up to - Everything in local and even a little distant peoples sensors Again to give Defenders a chance to see this threat and act accordingly With all the Categories i'f specified you could allow such a weapon to exist by using a combination of the things I've said, Yeah Planet busting weapons Screws other a lot of people building.. but if it can only be built in Certain zones? that is fine. And if everyone instantly knows when one of these is built they can counter attack it before it becomes a true threat. Encouraging large scale PVP In PVP zones, Because if you like PVP and built a large base or even a City in a planet and some one near by who is not friendly builds a planet buster? You Are going to do what ever it takes to stop that. I'd Personally like to see such a weapon could make PVP interesting and can be a real game changer in politics And in galactic balance of power.