the XXI was considered by many to be a revolutionary new submarine.
As we all know, many revolutionary new technological developments - especially those developed for military use - are on the drawing board for years sometimes before a working prototype can be developed.
even then - the "final battle ready product" might not exactly be "final" or "battle ready"
The XXI was plagued with problems which required a lot of post-production time to fix and the war ended, or the allies captured ports before most of the XXIs that were built could even be commissioned.
take the II and VII series subs which were available in the mid 30s... they had several years to work out "bugs" before seeing full scale combat.
I hate to say it but in the shallow waters of the english channel - no submarine at the time would have been able to interdict the allied landings and flow of men and materials into the European continent. the water is just too shallow and allied ASDIC was just too good at that point in the war.
any way you slice it, there was pretty much no way the XXI would be the saving grace of Germany during world war two no matter who they put into command of one.
as with several other things... the XXI was just too little too late