They're going to get locked out eventually. This is a scorpion-and-frog situation. It's in a major music publisher's nature to screw everyone else they do business with.
So anywhere one could say "they might get locked out if they do this" could as easily read "they might get locked out sooner if they do this".
Another advantage Spotify has over traditional music publishers is discoverability. The major publishers make most of their money on the most listened-to recordings of all time. Artists out on the long tail, with good product, but no hits, or in a narrower genre niche, get worse deals from the major publishing houses, and more to gain by bypassing them and going indie. If they can get a recommendations engine to introduce indie artists based on likes and dislikes from mainstream artists and pop tracks, and to occasionally sub in covers and style-shifted recordings, the consumer will likely accept it.
I don't need the Rolling Stones, specifically, to paint it black. I don't need any boy band in particular, anywhen from the Monkees to BTS, to reassure me that I'm their girl, baby. I don't need exactly Whitney Houston to be the one to always love me. I might need Prince to be Prince, Beyonce to be Beyonce, and Bruce to be Bruce, but I can deal without, if I have to.
With the media consolidation in TV and radio, I can no longer effectively discover anything new over the air. Everyone plays all the same songs, all the time. I can turn on my radio and spin the dial all the way around and never once hear even proven star Bob Marley, much less Peter Tosh, Jimmy Cliff, or Desmond Dekker. I can spin it again and never hear more than a sample of George Clinton, much less James Brown, Sly Stone, or post-NPG Prince. I can spin it again, and fail to hear anything as metal as Metallica, as punk as Rancid, as emo as My Chemical Romance, as polka as Weird Al, as march as JP Sousa, as southern as Lynyrd Skynyrd, as jazzy as Louis Armstrong, as bluesy as BB King... all the stations play only the released-for-radio recordings that sold the most copies. And a lot of the stations play only the current top 40 for pop. If Spotify can get people to listen to stuff they haven't listened to before, they will end up liking some of it, too, and wondering why they haven't ever heard it before.