The reason I think this is a good time to add one destroyer is because Germany will know that their window of opportunity to attack with the sub fodder will be closing quickly. So they may choose to attack at even less favorable odds instead of allowing Allies with that extra destroyer to split off and take out those subs the next turn, or if destroyer already in range why not do that now and have the replacement destroyer take its place?
Germanys options with those subs are very limited if US fleet is in sea zone 8.
Either way you are likely reducing Germanys odds to successfully airstrike the fleet.
I am guessing the counterpoint to this is that the Allied player WANTs Germany to attack their fleet to kill aircraft and if they build an additional escorts maybe Germany won’t do that.
To me this would be fine if they don’t. Its still likely wasted IPC for the sub buy that does not end up doing anything then.
The extra escort puts Allies ahead in the number count (which is close if they have 50% odds) should Germany decide to answer with additional air buy or the extra destroyer after dealing with the German subs can split off to help support another fleet in another sea zone.
Based on the Allied players comment they were already conceding some tempo by not bringing transports forward.
Then final comments of concern about not being able to land in Finland after losing their fleet.
Ships are expensive and take time to rebuild securely and so landings are now further being delayed at a pivotal point of the game. Pressure is off Germany in the meantime. They have a window now to focus ground forces east again.
Allied player says they had follow up, however based on what I see they only have follow up to sea zone 13 not sea zone 3 which their transports are in position to move to but now cannot do so safely.
To me this is following a decision tree too rigidly. The 50% attack offer rather than adjusting that pattern to the board state.