bitcoins reward math. bitcoin transaction data is not 0.X.. its actually measured in sats.
to make the smallest unit subdivide even further requires breaking and bastardising many bitcoin rules.

if you look at the byte data(raw tx) of a tx of 1000sat,  its not 0.00001000  its actually 1000

EG if a legacy UTXO had 1000sat locked to the key. and a BStard transaction format was measured in msats
the then code would think that the 1000 unit of legacy is only 1sat (1000msat)

also the block reward. its not 6.25/2/2/2/2/2/2/2/2
its actually 625000000 /2
or more technically
100101010000001011111001000000
were the end bit is dropped every 210,000 blocks

if a new BStard transaction format of msat looked at
100101010000001011111001000000
it would convert is to not still be 6.25btc but instead 0.00625btc because the unit of measure has shifted by 3 decimals at GUI level

this means to cludge the code with more code to fix that. the current
100101010000001011111001000000  (6.25bt legacy)
which has 30 halvings left (30 bits to drop)
would need to be changed to
1001000110000100111001110010101000000000
with 40 halvings. meaning it also breaks the 20140 year cut off. and changes it to be 2180 before supply depletes

..
also with 1000x more sharable units. that can be split and shared around. this also changes the scarcity.
EG no one cares about only 190,000 tonnes of gold. if there is enough gold dust for everyone to have, its not scarce because everyone can have some gold