Sure, legacy software is still catching up to multiple cores. Postgres is much better now or you can use SQL Server instead. None of them are native multi-master anyway so scale up is still better than scale out unless you only need reads.
I am not sure it is correct to call Postgres or MySql "legacy software"
To me, legacy software would be something like using Novel PeopleManager 2002 or WordPerfect 7.
Postgres 10.0 is < a week old, and is a perfectly fine path to go for a brand new application. You are fooling yourself if you think you should just throw every new application in MongoDB or Cassandra because they are "web scale".
If you are using "Legacy Software" to mean "software that has existed for a long time", then I guess sure. But there are many pieces of software that are very new, and could benefit from a single instance with a lot of cores.
The most common use of "Legacy Software" is "Old crusty stuff which needs to be replaced", which is NOT at all the case for MySql or Postgres.
It seems you've misunderstood my comments. Perhaps read them again? I'm saying that it's easier to have fewer more powerful servers than many more smaller ones, but even then some software designed for single-node operations still has problems utilizing all the cores (like postgres until recently). I never mentioned mongodb or cassandra.
Yes, Citus is good stuff. We use MemSQL though as it's a better fit for our (data warehouse) needs. There's also TimescaleDB, PipelineDB, CockroachDB, and more, lots of interesting options today for distributed "newsql" databases.
