The cases where Go is significantly faster than JavaScript are vanishingly small.
> whereas rewriting it later would be a massive undertaking.
This is vastly overstated IME. Porting existing code as-is between languages is actually pretty easy.
> This I very different to say starting a with a micro service architecture which imposes relatively high overheads,
Disagree. You're imposing a huge overhead on hiring (if you want people with both languages) or on people's ability to work on the whole system (if you're happy hiring people who only cover one side or the other). Debugging also gets a lot harder. There's essentially twice as many tools to learn.
Yep. We always implemented (parts of) embedded software in Java first and later in JS and then port it. If no additional functionality is added, this is trivial and saves a lot of work and errors as you already tested, debugged and fixed the logic parts.
This I very different to say starting a with a micro service architecture which imposes relatively high overheads, with little benefit as splitting up a well designed monolith is easy.