Many early stage startups can't afford even a single salary. By the time enough cash is on the table to consider those kind of purchasing decisions, the initial round of architecture decisions have typically been made, and been cemented into the initial version of the software.

E.g. I've worked on more than one startup where the first angel investment wasn't on the table before 6-12 months into development, and where that first round in some cases was below $250k

On top of that you also have the issue of finding someone that knows it, and the associated staffing risk that comes with that (yes, I'm sure you can always find someone, but at what price? there are place I can go where I couldn't throw a stone in any direction without hitting someone that "knows" Postgres or MySQL or both sufficiently well to be an acceptable tradeoff)

In many startups the tech choices end up being made not just based on what fits and what is affordable, but also based on what you can find affordable people to work with (including e.g. co-founders or other people willing to do initial work for equity) - sometimes that can lead to niche tech getting used. But far more often it means picking from a small set of the most common alternatives.

After that comment I'd bet good money you've never started a company or worked full time at an early stage startup.

Recommend you take one of your ideas and sketch out a back of the napkin first year plan. I bet it doesn't include using 250k of investor's seed money for a database.

More like the salary of two or three employees.

please do introduce me to your generous VC. At current low interest rates, building a stack that is reliant on KDB, will cause a large valuation hit because you're effectively locked into a negative -250k/year cashflow in perpetuity. At a 5% discount rate for example that's more than 5m USD of inflexible negative NPV right up front. Look there are fintech applications where this will be seen as fine (right tool for the job might be the big difference between success and failure) but you'll admit that it's a big ask for the less conventional "disruptor" style businesses which may not have big ticket upfront funding.