There are many aspect which prevent you from doing it, but mainly it's because
1. Community support.
2. How do you make your change backward compatible.
Nobody can prevent a soft fork from taking place due to its backward-compatible nature but also the minority chain (assuming the soft fork was minority like if OP did it on his own) can easily be attacked by breaking the rule they enforce and forcing them to reject the block containing such transaction effectively forcing them to split.
While it's theoretically possible, you also prove such attempt isn't practical and likely will failed or ignored.
the latter group, nodes which have no part in setting up a block candidate for mining devices to try their luck finding the magical nonce, could go back in the version number almost down to the original Satoshi client
It's still far from original satoshi client, AFAIK the oldest Bitcoin Qt which can be fully synced is either Bitcoin-Qt 0.7.X or 0.8.X.