A giant pool? Sorry, not really. JS skills are highly sought after, and finding good, experienced front end developers is very hard. Inexperienced developers are easy to find in any language, including your favorite one, whatever that is.
The "cheap, generic, interchangeable, fully replaceable" part just sounds bitter and vindictive, no need to respond to that.
I will concede that I am a bit bitter. It’s frustrating to watch the industry push ever harder to render specialized developers unnecessary.
As for quality, correctly accessing a devs level of competence can be very tricky, regardless of the required skill-set. With web devs, its even more tricky because an "over-qualified" desktop/app devs can do just as much damage in terms of technical debt as a poor quality web dev. A webdev is also a specialized developer, but due to the low entry barrier, many don't realize this.
Android dev is messy business, no question there, especially if you care enough about experience quality to test across a broad range of phones, OS versions, launchers, etc. It’s no wonder why the average Android dev team is 2x the size of the average iOS dev team. Not much you can do there but hope Google makes an effort to clean up (and it seems they are, albeit slowly).
Even with all these platform specialists accounted for, companies like Slack and Spotify aren’t hiring any more engineers than they would otherwise. The only difference is instead of hiring specialists, they’re hiring hordes of web front end.