In the case of cryptocurrencies, the answer is likely a combination of what miners and users can support. The goal is to be large enough that we don't have throughput problems and block space doesn't become a bidding war.
It's also worth mentioning that this doesn't mean the end-users have to be able to run nodes themselves or on mobile phones, since that makes use of a process called Simple Payment Verification (SPV) where they only process block headers and get the blocks they are interested in, but still verify the consensus of the network.
But, it also depends on what you see cryptocurrencies as. If you want them to replace traditional cash and create a "digital cash" world, then you want to see them accepted and used anywhere at a very large scale. In that world-view you have millions of transactions regularly and miners get paid with the sum of lots of small fees.
However, if you think that cryptocurrencies should only be used as a speculative asset or as a "digital gold" then you don't care much about it actually being transacted, and in that case fees as high as $50/tx are fine because you don't have any expectation that the world will be using it at scale.