It wouldn't? If I take the example overseas forts (like the many european holdings in india) that would be a gamer changer. Suddenly a indian power without naval supremacy would face the problem of either committing to a prolonged siege that may never end or an assault on the fort. And that situation is able to happen thanks to the usage of a supply lines. Now if it was a big army, so you navy can't fully resupply it, I would think its better to have some supply than no supply at all, so not entirelly useless (preferable than just not being able to do stuff).
Oh well, leaving as a debuff totally depends on what you want the game to be. If you want to make conquests way easier, like how it works now, then it makes sense gameplay wise for that case. But if you want the game to be not just a map painter, then a debuff isn't enough.
Even so I don't see how is that bad gameplay wise for non-naval powers to struggle to capture coastal provinces. One of the points I made at the beginning in the thread was navies didn't felt important in warfare. And if having a navy helps and can make you hold a coastal fort then it's exactly what I want to go for.
Lest's not forget, assaulting forts was a thing and artillery usage too. Of course all of this will snowball into the problem that eu4 sieges don't work at all for a system like this. Forts use magic manpower, artillery for some obscure reason isn't available in 1444 and assaulting forts barely makes sense and it's always bad unless the fort has less than 1k garrison. Then you gotta balance it out so its hard to supply coastal forts too... This is kinda why I avoided to suggest any idea of supplying troops using navy and suggesting forts could be resupplied in the main post, it snowballs and looks more like an idea of eu5.
Like I said I don't see Pdox making it so navies can be put on a supply mission due to it breaking the game. AI either getting stuck in never ending sieges and then peacing out after 5 years or wiping out their manpower and getting debt spiral with suicide assaults.
It's not that they will struggle, it would be next to impossible. You make it so you need ships or be a land super power to be able to take a coastal fort. England will have no issues with the 100s years war despite their last real world holding of Calais actually fell in only 7 days with no need for France to use a navy to blockade it.
Artillery does exist in 1444, it's just not that powerful or effective. Same way we have undermining which was common but time consuming and things like trebuchets, that's how you can get wall breeches without having any units of artillery in the army. They are representing those guns that started to see use on the battlefield but covering their added benefit during sieges.
Yeah assaulting them is bad and why it's currently broken, you need to change a lot more than just the fleet mission but also the entire assault mechanic. Attacking a fort was generally bloody and expensive but in EU4 it is just suicide.
Yeah, it's also skipping over that even during the Napoleonic wars there wasn't the top tier ships sitting outside of blockaded ports but would generally be light ships that could deal with merchant ships but were fast enough to escape any larger warships. France did manage to sneak out a few times to move ships from port to port during the war. Something more like the HoI detection system would help there to make smaller forces able to act out more.That's a good observation that I forgot, sea tiles in eu4 don't have weather conditions that would cause attrition to ships like land. In fact ships only take attrition if faced a specif coastal battery (which you can ignore by having maritime) or going outside of your naval supply range.