The idea that every system must scale to millions of people is so fiat.
Some things are just better at a smaller size.
Discussion
if the bandwidth and compute costs are the limit to scale then that is going to be a problem irrespective of grand scale, it's a problem at small scale
niche doesn't need to mean shitty and poorly optimized, fine, first beta can be slow and shitty but it should be optimized as soon as it's proved working
and don't get me started on the problem of bad specifications