If the number is so important why cant we replace the lost coins?
Because it would undermine confidence in BTC and it's not necessary at all. Just split them up!
how would it undermine confidence if the coins will not be undermined but just mined?
The Value of coins is under the premise that there currently are only about 12 Million coins and that there will be only 21 Million. Also every lost coin is basically priced in.
"Issuing" More would reduce the value of every coin, also it would undermine the confidence that the new Max, truly is a max.
Luckily, it's simply not possible without the agreement of the users.