I'm not a great fan of the German navy, but I still have some somewhat more constructive opinions on naval affairs. Admittedly, naval affairs are very technology-dependent, and I am likely lagging in naval technology understanding by a decade or so. This isn't necessarily bad - some navies lag by more than a decade.
My basic charge against navies - particularly the German one - is an overemphasis on platforms. They like their ship hulls, their officer career opportunities and so on. They would resist a move to unmanned platforms, boats instead of ships, land-based systems instead of ships and so on with as much determination as horse cavalry officers resisted the acknowledgement of their obsolescence. Or maybe they do already.
So here's my opinion, which navies would abhor:
Maritime transport protection in wartime
(1) along secured lanes, following this model and improved by land-based fighter cover and mine hunters
(2) beyond those lanes in convoy format, possibly by means of armed merchantmen which carry containerized ASW* and AAW solutions even involving AEW&C helicopter support that enables the use of long-range air defence missiles (SM-6 and similar) even against low-flying targets
(3) harbour protection against divers and unmanned underwater vehicles
Defence against amphibious invasion
(1) land-based defences (sensors, missile launchers, communication)
(2) land-based combat aviation
(3) the army
None of the above requires a single warship more expensive
than a slow and small coastal minehunter.
Navies would hate it.
Frigates and destroyers would no doubt still be useful, but only as escorts for the typically fast-moving aircraft carriers, in carrier strike groups. These again make sense primarily to project air power to areas far away from friendly air bases - which in itself almost perfectly excludes that this might be about national or collective defence. The U.S. defending Taiwan is about the only exception, and a most unlikely one. The expenses for a single CVN would suffice to greatly bolster Taiwan's defences and defence infrastructure directly.
That's why I think carriers and dedicated amphibious warships are offensive platforms, grossly inefficient for defence. This is also why I am so sceptical about the navies' obsession with super-expensive platforms with high operating costs. Containerised modules for armed merchantmen and coastal-based defences would be totally against the bureaucratic self-interest of navies, but they might be the much more cost-efficient approach.
The typical claims that navies protect maritime trade is ludicrous in context of a great war (against other great powers); the numbers of warships are grossly inadequate for this and many warships would be busy escorting offensive warships such as carriers and amphibious warfare ships. The German navy has a few high seas-capable frigates, but those wouldn't even suffice to protect the traffic lane in the coastal waters between Hamburg and Rotterdam only.
*: The loudly emitting low frequency sonars would enable long-range torpedo and missile shots by submarines at such a convoy, though. This may be an unacceptable problem if the guided munitions are effective enough. An answer might be to use a screen of sonar emitter boats in a multistatic LF active sonar network. The long range shot problem may be unavoidable with both armed merchantmen and regular warships, though: The long range of those guided munitions coupled with the loudness of civilian transport ships may enable effective shots from beyond any practical screening radius..