Upgraded planets already take forever to destroy without alot of wasted resources with siege frigates, why would you want it to take longer?
Because planetary mitigation would happen in complete, desolate isolation from all other potential changes to the game.
With mitigation, planets will gain a minimum amount of time that they can be expected to hold out. No matter if you send 2, 5, or 20 siege frigates, the planet will be able to survive a few volleys due to mitigation.
Currently, 10 siege frigates are 4 times as good as 5 siege frigates. Think about it. Double the siege frigates take twice as long to kill, and are doing twice the damage during the time. With mitigation, 10 siege frigates will still survive twice as long, but will have diminishing returns on damage. This means they won't be gaining exponential planet killing potential as you add more, the curve gets flattened a lot more.
With this change, siege frigates gain a LOT more flexibility in being balanced. This means siege frigates can be buffed without causing undue issues in the game. That's the main idea, to provide a comfortably sized balancing platform for a siege buff.
You also have failed to realise that while spreading out the forces would in theory kill them quicker due to less mitigation, those spread out seige frigs would die even quicker due to lack of protection...
Yeah. So what? If you are pushing so hard that you can secure multiple systems at once, good for you. But if you want to kill a planet FAST, you will have a harder time than before.
Capital ship abilities would have to be tweaked such that they also are affected by mitigation, or they'd race miles ahead in siege power. Percentage damage such as hysteria and drain planet would need changes so that they (drain planet) can't be used to gain undue benefits.