Scalable software tools (mongo, elastic, rabbitmq, kubernetes), unlike the name suggests, do not guarantee scalability. Scalable tools designed to span across multiple threads across multiple machines, possibly across different networks and even multiple data centers, allowing for potentially higher ceilings. The kind of scalability that the vast majority of systems do not need. Database design, backend infrastructure is what's crucial.
Tennis
As a recent beginner dabbling in tennis, I was in the market for a racket. I bought one for $30, not $200. It could've been better, but as a non-professional, the marginal improvement is not worth the 6x investment.
But we don't apply the same logic when making software decisions. Scalability comes with incidental complexity, more friction, a learning curve, set-up time, difficulty of maintenance, etc.
Relevant: Don't Scale by Paul Graham.