You say the US can donate say, 1 loaded carrier. Just when does the US move this carrier to join the British Fleet? That would be AFTER the Japanese do their turn. Do you see the problem with counting on US fleet support for a moving British Navy in the face of Japanese air threats?
So now all you need is a few Brit fully loaded carriers and destroyers and a few US fully loaded carriers with destroyers. So now either countries fleet can survive as an independent fleet in the face of Japanese air threats. The moment you think this occurs, Japan could add 1 more air unit that TWO independent fleets must defend against.
Do you ever plan on having the US and Brit fleet move? If you do, then you must invest more in fleet defense in the face of a Jap air threat that moves between when the US and Brit does.
You keep ASS U ME ing that the Germans just sit back and turtle. With Fortress Europe, Germany can be more aggressive as the allies have spent more on fleet defense and less on land units that actually are what get you territory and more IPCs. With 4+ Jap fighers on WEu PLUS whatever Germany can spare on defense and allied transports and landing units in smaller numbers as they invested more in ACs, Fighters and destroyers it becomes easier for Germany to lean forward.
If you have an allied fleet which cannot operate independantly in the face of a Jap air threat then the axis is more free to to manuever knowing you cannot split your fleet in two.
Perhaps you ought to challenge an experienced Fortress Europe user to a game to see how well it works. Trust me, I’ve been playing since the 80s. I was gloating at first when I saw that Jap airforce in WEu. I thought, what a waste of Jap power. Then when I wanted to drop some forces of in Algeria and some in Norway and purchase new fleet and move up last rounds builds of fleet I realized what a balancing act was required. It constrained what I wanted to do. You might never need 4 loaded carriers in the face of only German air. You might need 4 loaded carriers if you wish to move the allied fleet in the face of a decent German and Jap air threat.
The premises of Fortress Europe is to cause the allies to invest far more in fleet due to the Japs being able to hit either fleet as they more independently of each other. It has gotten to the point my friends feel the allies need a bid to overcome fortress europe. I think they and myself just need to see an example of somebody beating fortress europe. I am told it occurs, just never in this forum. TripleA is apparently where the ‘professional’ games occur, I have yet to play there or even review games played there with/against fortress europe.