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?
People don't seem to be getting this:
If you own bitcoins, what is important is the % of total bitcoins (that will ever exist) that you own. If any change occurs which preserves everyone's % of total bitcoins, then it should have no effect on the value of your holdings. Adding more decimal places preserves the % of bitcoins that everyone owns. Deciding that more bitcoins should be mined changes the % of the total bitcoins that everyone owns, and is therefore equivalent to theft from existing owners.