Maybe "worked out great" means that they got a ton of publicity and traction because they were open source, and now they are ready to abandon open source so they can monetize the traction they gained from being open source in the first place?
Sentry certainly doesn't have any obligation to continue being open source- the old code is still open source- but these events make me question any companies commitment when they claim to be open source. It seems that you can't take any company at their word, even when they publish multiple blog posts about how "committed" they are to open source- because in the end, many companies will do what Sentry did and will dump open source the minute they have gotten what they want from it.
[https://blog.sentry.io/2019/02/14/sentry-thrives-open-source..., https://blog.sentry.io/2016/10/24/building-an-open-source-se..., https://blog.sentry.io/2015/06/30/driven-by-open-source]
And if there was any doubt about Sentry's willingness to use and abuse open source- months after this announcement, they continue to advertise being open source on their site, lying to the community and their customers. https://sentry.io/_/open-source/
Sentry doesn't have any obligation to continue to releasing new code as Open Source, but they should stop lying about being open source.