HA is now pretty good on Postgres.

As for scale... Just use a larger machine. This works for regular transactional data until you're at something like Amazon scale.

Edit:

Think about this, suppose that you store 1 megabyte of data for each of your customers. So if you have a million customers, it's just 1Tb. And these days, you can have a server with 10Tb RAM delivered overnight. Although you might have to sell your firstborn son (offer applies only to royal families) to fund it.

A lot of sharding/no-sql/... development happened in the late 2000-s when computers were about ~100 times less powerful than now. You _could_ get a system with 10Tb RAM in 2010, but as a specially-designed supercomputer.