We could keep libdb-dev for the fork keeping the current license andcreate a new set of development packages like libdb6-dev for the AGPLv3 code with or without switching to an upstream different from Oracle.
And if you read the thread backwards, that's exactly the thing we want to avoid. For the sanity we want to have just one active version of BDB in the archive (we usually need the old tools).
Database files created with different versions might be incompatible (depends on the features you use, but usually the on-disk log format changes with every release), and you certainly don't want to have f.e. python compiled with BDB 6.0 and Perl compiled with BDB 5.3. The would be mess we were able to avoid in wheezy and I don't want to have it back (squeeze had a random mix of db4.7 and db4.8).