While I agree that this is a wonderfully simple case the main problem is that it isn't always that easy. As long as you get get away slicing tables into different "shards" and aren't to join-heavy it isn't too baf. However it still adds complexity to the application code that is a constant maintenance cost. And eventually if you keep scaling you will get to the point where you need to shard a single table, and that is very painful. Both in the risk and difficulty of the conversion and further ongoing development costs.
I do dream of the day when distributed databases are the "default" for new projects. Likely running just a single instance at the beginning. But then you have a seamless path forward when (and if) you need it. Even at medium size three small nodes will be easy to manage and give easy HA and allow you to do version updates safely and with no downtime.
I don't think we are there yet, but there are a few contenders in the running but Postgres is tough competition. Its years of stability and predictability give it huge points even if it has the downsides of a centralized system. But I think the turning point is steadily approaching.