Ideally yes, but as we've seen time after time, charity as a business model doesn't work with corporations. I think the coreJS maintainer would be fully justified to take another job, and then work on coreJS as a hobby as much or as little as he likes. Nobody needs to martyr themselves so that a bunch of billion dollar advertising companies can save a buck.
Maybe in the future we decide to finance core open source projects as public infrastructure through taxes, or those billion dollar corps set up some kind of foundation to fund them, but for now the whole situation is highly dysfunctional and one cannot blame maintainers who put the financial interests of themselves and their family ahead of producing open source.
Of course they won't volunteer to maintain a massive project that they depend on, when someone else is already doing the work.
But you can bet the minute core-js goes away, the community will pull together to maintain or create something new, purely out of necessity - that's the main motivator. Hopefully with a more distributed model that doesn't overload a single person.