1. The best programming language is the one you are probably using at the moment, although it assumes you would pick the right langauge for the job at hand.
2. Mixed language projects are problematic.
3. The ability to migrate ownership of a codebase and bring new engineers up to speed is very important.
I think we all recognize these things as self evident but, I certainly, have not always had the discipline to keep front and centre.
Strongly disagree with this point of view. Specific examples: php is good at stitching presentation layers, python is good at number crunching, ml and ai, nodejs is great at all things web, c++ is awesome at fast memory intensive low level programming, and so on. Use the right language for the task at hand, and as a programmer don't shy away from expanding your knowledge area. Instead of using a shiny new framework that does the same thing but different, better use the right language. A programming language is just a language and each has its own edge over the other.
You're skipping over the cost, which is in reliably and efficiently passing information back and forth between languages. It's generally some form of serialisation. Occasionally both languages get to run in the same address space which moves the complexity around a little.
There's also the human scale drawbacks of having predominantly C++ devs write JavaScript or vice versa when people should touch the far side of the language barrier.
As usual, there's a balance to be struck. Only the Sith deal in absolutes
