Seems quite naive to think they'd be affected in any way by the tiny intersection of users that are both yt-dlp users and premium subscribers boycotting them...
To buy premium to support creators.
Once yt becomes hostile the deal between me and yt is off.
The concern is likely that if they let it become too easy the small minority becomes a large majority and the ad business becomes unsustainable.
Consider the ease and adoption of newsgroups in the early 90s vs Napster/Limewire later and the effect that had on revenues.