Older blocks may no longer be considered valid blocks under new rules
- I can't believe that, older blocks are valid because they exist and have been confirmed everything else would contradict immutability.
If the version numbers did not exist, then we would have an issue where some v2 blocks are no longer valid under v3 rules and the clients would get all screwed up because they have historic blocks that don't validate.
- The version number is a variable(!), every miner is free to choose whatever she wants. If what you say is true, somebody could create quite a mess with sending an old version number. In such a case we should remove it as bug urgently.
Version numbers also help to facilitate miner voting. Miners voted for using BIP66 rules by producing v3 blocks. BitcoinXT nodes vote for BitcoinXT by producing blocks with 0x20000007 set as the version.
- They can also facilitate forking, again, there are clearly different versions being developed right now.
Maybe, we have only a very narrow window open to make such changes to the block layout. Once there are 5 different implementations out there it will be impossible to even change such a small matter.