which can break bitcoin in many ways about backward compatibility and forward compatibility of reading transaction values and reward rule amounts
How can it break it?
Once we came into this agreement of four more decimals, the code would change, but Bitcoin wouldn't break. Satoshi (sat) wouldn't be the base unit (as it is in the source code), but another one which is a thousand times less than that. Transactions signed after this change could have a prefix which would indicate that the user signs based on the new milli-toshi base unit. Any transaction's amount which was signed without this prefix, like the ones before the change, would be considered as a thousand times greater.
the binary rule.. the real base rule the buinary measure will be broken
you are clamoring onto ideals that the hard rule is actually the soft rule of the user visual of btc.. without realising or admitting that the real rule is binary.
changing the binary of 6.26btc means that old nodes see the new measure as being more then 6.25 btc.
and new nodes see the old 6.25btc as being less than 6.25btc
and it then requires alot of cludgy math to translate and hope to not break the binary rule by having cludgy math express numbers different at human interface level
if you think you can have cludgy user interface xcode to ide the rule break than you are pathetic.