I agree with your two first paragraphs. I want to share some elements to complete your outlook.
Javascript, despite having monopoly on the client side since 2007, only saw its usage exploded with Node and NPM around 2013. Java applets and Flash were long dead by then. So it seems that JS success is only partially correlated to its hard-won monopoly on the client side. My intuition is that JS ecosystem cannot be that bad and win so many hearts. Using your words, it seems JS won based on some merits.
A few others clues: Hot reloading, fast build time, good debugging and profiling tools, lots of packages, plenty of already answered problems on stackoverflow/github, progressive complexity, multiple approaches possible from FP to OOP, particular attention to the developer experience (error messages, colors...), good set of linting tools with Typescript or Flow, easy to deploy/deliver, works to make desktop/mobile/web/VR apps, customizable build accessible to mortals, good enough package manager, evolving language that understood that it can always do better, welcoming and accessible dev community, mature networking APIs, good runtimes implemented by different companies...
I've been in this space pretty much throughout the whole period. Applets never took off as a serious option. They peaked briefly around 2000, then flash took over and by 2005 Ajax happened and JS took over.
As for build times, modern javascript is like having all of the disadvantages of a tedious build process without most of the advantages. You are spending the time, yet still don't have static type analysis and exclusion of whole categories of bugs. The reality is that, mostly for political reasons, javascript was the only thing that ran in a browser and after particularly MS (but certainly not them alone) ran into trouble with security issues, nobody really dared to do much else for a long time. Plugin APIs were deprecated, proprietary browser specifics were abandoned, etc.
So, what's happening now is a long over due correction where people compile to some heavily optimizable subset of js called wasm. As soon as that becomes commomplace the whole argument for javascript needs to be revisited, pretty much for the first time this century.
Javascript, despite having monopoly on the client side since 2007, only saw its usage exploded with Node and NPM around 2013. Java applets and Flash were long dead by then. So it seems that JS success is only partially correlated to its hard-won monopoly on the client side. My intuition is that JS ecosystem cannot be that bad and win so many hearts. Using your words, it seems JS won based on some merits.
A few others clues: Hot reloading, fast build time, good debugging and profiling tools, lots of packages, plenty of already answered problems on stackoverflow/github, progressive complexity, multiple approaches possible from FP to OOP, particular attention to the developer experience (error messages, colors...), good set of linting tools with Typescript or Flow, easy to deploy/deliver, works to make desktop/mobile/web/VR apps, customizable build accessible to mortals, good enough package manager, evolving language that understood that it can always do better, welcoming and accessible dev community, mature networking APIs, good runtimes implemented by different companies...