I pretty much agree with everything you said. I too think the Enterprise commissioning/decommissioning factor is more political than practical. However, if that is indeed the case, then I would also like to think that the previous decommissioned Enterprises that weren't destroyed (including the A, B, and E) were recommissioned with a different name and registry, since there's nothing inherently wrong with the ships, and I wouldn't want to go under the assumption that they were all lost or destroyed during their active service (I always had a theory that the USS Lakota seen in DS9 was actually the former Enterprise-B.) Unless they were all immediately put into a museum, which is also a possibility.
The one oddball here is the Enterprise-C. At some point the B either gets decommissioned or destroyed, and the C is built as its replacement. However, the Ambassador class didn't seem to be all that popular, and when the C was destroyed in 2344, Starfleet didn't replace it with a new one for 20 years. So that one I can't explain.