1) Shouldn't be a thing as ships last at most around 30 years and don't last trough the centuries (shipbuilding capacity might be able to simulate something, but not all)
Sure but I guess I'd say that upgrading your ship capacity doesn't represent you actually building a standing fleet. Most of your ship building comes from upgrading your economic centers, namely harbors and ports in your towns. So I don't think it's a stretch to say that building these upgrades merely reflects your ability to find trade ships to press into service. Again, the upgrades are ports, it's not actually called "build fleet." Moreover, I believe the technology you need to upgrade your harbors is actually the "Trade Practices" technology, which is just more evidence that what the upgrade is doing is that by increasing the availability of port facilities, you're increasing the amount of merchant ships available to press into service.
Also, given the fact that ships have a huge gold upkeep, I think it's fair to say that you're paying for the merchant ships, or because you've put merchant ships into service, you're losing a bunch of revenue because you're hurting trade in your provinces.
3) Should also not be a thing as most if not all ships of most rulers should be mercenaries/ merchant ships pressed into service.
So I guess I sort of covered this above. But also, is it really so far fetched that you wouldn't be able to press enough ships into service from the merchant fleet available, so you'd have to arrange to get ships through some other means? Like, wasn't an entire crusade delayed because they wanted to hire a mercenary fleet from Venice but there were difficulties paying for that?
4) A decent point although it is mostly made up for by having a combat penalty after landing.
5) ^
6) ^^
Right I guess I disagree that the after-landing combat penalty makes up for it. 2 reasons for that -
First off, I just think the tactical decision I mentioned there are for more engaging and far reaching than just a landing penalty.
Secondly, it can't really "make up for it" when the landing penalty was in CK2 as well. It's not like we traded one mechanic for another - CK2 had these tactical decision + landing penalty, and CK3 just has landing penalty. Getting nothing in return doesn't make up for it.
Just for those who were unaware - there were 2 different "landing penalties" in CK2. First, disembarked troops would disembark with only 50% morale (even if they had 100% when they embarked), so it would take them a while before they were able to fight, lest they rout due to very low morale. Secondly, if there were troops stationed where the troops were landing (so a battle was started immediately upon disembarking), the attacker would have very severe combat penalties - this combined with the morale meant that you could often win against disembarking troops 3-5x your number.