"Basically any fundamentally correct buffer encoded as message A will decode successfully as message B for any B."
This is incorrect. I suspect you're overextending proto3's treatment of unknown fields to include discarding incorrectly typed fields too. If A has field 1 types as an int, and B has field 1 typed as a string, an A message with field 1 set will not parse as a B message. However, if the A message has no fields set, or sets a field number unknown to B, that could parse successfully with "leftover" unknown fields.
This is incorrect. I suspect you're overextending proto3's treatment of unknown fields to include discarding incorrectly typed fields too. If A has field 1 types as an int, and B has field 1 typed as a string, an A message with field 1 set will not parse as a B message. However, if the A message has no fields set, or sets a field number unknown to B, that could parse successfully with "leftover" unknown fields.