Is stackoverflow essentially a monolith? I remember reading about the single database setup several years ago, I'm surprised that they have been able to keep scaling vertically.
No groud up re-architecture, no microservice complexity, no crazy high cloud bills for exotic high power VMs. Just upfront capx for your own big iron and the skill to run it.
In terms of opportunities to add complexity: I'm surprised there is no mention of a data-platform or any model training for ranking, etc.
To reduce components, it might be possible to combine ElasticSearch and MSSQL into just PostgreSQL which has awesome text indices. As MSSQL performs very well and presents a somewhat esoteric SQL dialect this could be an expensive project.