So why miners can't be satisfied with just the coinbase earnings? no fees for users = more adoption = BTC value goes up.
First of all, transaction fees is used as a mechanism in bitcoin to deter spam. If transactions can be made without paying transaction fees, then there is no cost to the user to create spam transactions that will eventually overwhelm the network. So, the inclusion of transaction fees is a necessity for network health.
Secondly, it is not necessarily true that "no fees for users = more adoption = BTC value goes up". Paypal incurs zero fees for friend to friend transactions, does everyone own a Paypal account? Would onboarding many users that do not pay transaction fees cause Paypal shares to rise? Probably not.
Finally, a fee market for bitcoin transactions is a good thing as miners need to be compensated for the work they contribute to generate blocks. As block subsidy tends to zero, the proportion of mining revenue derived from transaction fees must rise or at least reach an equilibrium where the mining revenue is equal to the security cost of securing the bitcoin blockchain.