no dought mining will be very popular and developed when miners will get the transaction fees but its hard to say that do they will mine the last coins in 20year or not because in middle of it moee inventions can happen too
If mining will be less profitable than it is now (read TX fees don't rise up to a block reward amount as of now), how can it become very popular?
Inventions affect hashrate but not the speed with which coins are mined
I believe the block size should be increased soon as that more transactions can be done in a faster manner.
Increasing the block size would evidently decrease fees since there will be less competition between the senders of funds for the inclusion of their transactions in the newly found block, which consists in increasing the fee size (the higher the fee, the faster the TX will be included in one of the new blocks). Miners surely won't like that...
Thereby, the block size is not to be expected to get increased any time soon