I think this still returns us to the same problem though.
The issue isn't motivation on the part of the defender. The problem is that you can build armies in virtually limitless supply. The two bottlenecks on building armies (pops and minerals) don't have any functional effect. You can build more armies than you'll ever need without bumping into either cap.
So once you win the space battle and take the system, you can wear down any planet through sheer attrition. Ground mechanics like army composition, number of defenders, etc. don't matter in the face of numbers. You can throw your army at the planet, lose them all, then just rebuild. Rinse and repeat until you win.
The front line system of galactic geography emphasizes this because it makes it almost impossible to intercept transports. Except in rare circumstances, a captured system will always lie behind the front line of battle. So it's not like you can stop the transports or cut off the planet. (Because the proper model for naval combat in space was clearly WWI-style trench lines.
No other model was even conceivable.)
Right now the only way to stop that process is to take the system back. Which brings us back to the fleet. If you can win in space, you can take the system back and prevent them from taking the planet. If you can't win in space, they can keep the system and spam armies until they take the planet too. So the space battle is the only part that matters not because losing the planet is unimportant but because keeping the planet is impossible if they can hold the outpost.
War exhaustion was supposed to push back on this, but in practice WE is also almost exclusively based on fleet strength. I've had many games where I literally held every one of the enemy's planets and star bases, but the AI refused to surrender because I hadn't yet annihilated the fleet. So you can toss armies into the meat grinder with little downside.
That's what needs to change though. If they want to fix ground combat, they need something that changes the basic logic of: Take the system, spam armies, win through attrition.
Personally, my vote would be:
- Implement a much harder cap on armies. The current system says that each pop can only support one army at a time. Give each pop a cooldown so that it's also X years until that pop can support a new army.
So if I build an army, it might get linked to Pop Alpha. Right now I can't build a second army until the first one dies because Pop Alpha can only support one army at a time. Maybe Pop Alpha should also have a three year timer, so that I also can't build a second army until Pop Alpha's cooldown has expired. To prevent micromanagement, you could have a simply tooltip saying "No available soldiers. X Days until next recruitment."
That would prevent the endless army spam. A defender could invest in real defenses, knowing that it's actually possible to win a defensive battle if they hold out long enough.
- Also fix war exhaustion and war score. These are two separate values (for some reason) that control white peace and surrender. Both are almost entirely defined by the fleet and shouldn't be. At least for many ethics, losing lots of armies should at least spike WE and should probably heavily influence WS too.