I get his frustration about large companies using core-js and not contributing back, but he is at least partially sleeping in a bed of his own making by licensing core-js as MIT.

The founding idea of Free-Software was always that everyone contributes a little and by using a each others work you can grow a foundation that is larger than its individual parts. For that to work you should use a copyleft license like the AGPL and treat any library you develop not as an end in itself, but rather as a tool for something else (and this something else might or might not be what you make a living with).