Instead, we should focus on more pressing matters like mainstream adoption of popular cryptocurrencies like BTC, ETH, LTC etc at present.
This is bitcoin discussion which is one of the bitcoin boards on this forum, not altcoin board. Ether, Litecoin and other cryptocurrencies that are not bitcoin are altcoins. Altcoins are not perfectly decentralized like bitcoin. If we should talk about adoption, it would be better to talk about bitcoin adoption.
If the miners shut down,because the fees are unprofitable,the hashrate/mining difficulty will get adjusted,which means that mining will become easier,which means that it will become profitable again.
It would become profitable again but the mining hashrate reduce which means bitcoin blockchain will become less stronger and more susceptible to 51% attack. The hashrate may not be reduced to the extent 51% attack will be possible, but I want to indicate the disadvantage of reducing mining hashrates.