The solution for BTC appears to be off-chain scaling, such as Lightning Network.
The end of mining rewards is beyond our lifetimes, but I wouldn’t be too surprised if there’s a successful hard fork to continue the final 1 Satoshi reward indefinitely.
If it's found that deflationary really won't work, and it's genuinely hurting the usage and adoption of the currency, and it's in the current users of bitcoin's best interest to do so, it can be turned into an inflationary asset.
Flaws can and are fixed in it, and because those changes can't be pushed through by some appointed authority without overwhelming majority from all involved parties, you don't need to worry about this ability to drastically change being unfairly pushed upon you.
Difficulty, transaction costs, competition with other coins (and off-chain networks/sidechains), and the market price of bitcoin will all hopefully & probably settle around an equilibrium that is sufficient to incentivize enough miners necessary to maintain the security of the network.
If the transaction fees aren't high enough to support the current level of mining them some (but not all) miners will drop out (and the "difficulty" will adjust to compensate).
The result is transactions will continue to be processed, but the overall security of the network will be lower.
Lightning transactions will change all of this. By the time we are talking about large lightning transaction commits, we'll be talking about 3rd layer solutions. Bitcoin could be at a reserve currency level, where individual transactions are significant.