seriously, if bitcoin is not massively used (like thousands of transactions per second) by the time block rewards become really small, it can pretty much be considered a failure.
Bitcoin will become more and more a store of value. Why is that considered to be a failure, when this is by design?
Because, if there are not enough transactions, the miners will stop mining, and the network becomes less secure, and more likely to be controlled by a single malicious person.
I would personally consider this to be unlikely, especially considering how many additional merchants are accepting bitcoin and likely how many more consumers are adopting bitcoin.
Also miners would not completely stop mining, they would just have a lot of miners taken offline