The other ideal is the principle of free software being like software I would write myself. MIT and the unlicense are strong versions of this, where I can use them in (nearly) any way I would use software I had written from scratch. The BSL is the better model for this, because it entails paying for software that, in a few years, you will own in this strong sense. Open core entails no such guarantee, and if the company producing it went under, there is no guarantee that they would release the source code under a permissive license.
I think the impetus behind their decision is that they didn't observe a strong open development community, so they didn't feel they would impact that too strongly by using the BSL. The really interesting license would be a combination of both, that is having a truly open core, then licensing everything else under the BSL or equivalent. This is probably avoided for the complexity factor, but would probably entail the best of both worlds.
Given that Sentry is primarily the sole contributor behind improving and advancing their own product, making it more sustainable directly translates in being able to have more resources into making a better product whereas having their efforts stolen and repurposed by a competitive AWS/Azure hosted solution would directly make it worse. They'd be pouring their millions into competing against themselves whilst the cloud providers can use their marketing + monopoly network effects to reap all the commercial efforts off Sentry's investments.
The only losers here are the cloud oligopolies and Sentry's hosted competitors, Sentry ends up with a more sustainable business model they can invest in making a better product, which benefits their product and all their customers.