alt Hacker News> microcontrollers
is a marginal niche, but everything else can be handled by one DB. You think we have PG with extensions (including duckdb) already covers most of the ground, making something like that distributed is also feasible task (there are projects), architecting system to make core engine embeddable is also feasible task.
Even microcontrollers would merely require custom compilations that turn off features.
There are no technical reasons for having more than one codebase, only economical and coordination-related.