I've seen people say that the limit can be increased if needed, but im not sure.
Not necessary, 21 million bitcoins is enought and even if only 1 million bitcoins left (20 million bitcoins lost), it would be enought for Bitcoin to work normally
Raising the limit of 21 million BTC is not possible. (not suggesting that is what you're implying ..just that it is more than just not necessary -it's not an option) The only way to increase coin availability is to divide it into increasingly smaller fractions of a coin; .00000001 is currently the smallest unit, however, it can be divided into smaller units if demand justifies it.