So if you got the rights from the BSL but the part about the code becoming Open Source you'd be happy with right? I think that's a completely fine attitude to take, and sure, all other things being equal, code licensed under the BSL is better than not having access to the source at all. But, given (at least) the majority of BSL projects were under an open source license, with external contributions, the spectre of "I have changed the deal, pray I do not alter it further" is raised. Additionally, if you read the actual post by Hashicorp, they do sail quite close to the wind in terms of implying the BSL is open source, so had they been more careful in their language (or even in their CLA), then people would be less annoyed.

The reason I'm personally annoyed at the attempt to drop the requirement for no restrictions on use is because it is something some academic software does (things like "if you use this code, any results must be shown to me before publishing and require my approval", which as you can imagine isn't great), or with rules around benchmarking (which some DB companies have been known to do), and it's significantly harder to draw the line between something that would allow the BSL, or the above two cases, than it is to require no restrictions on use.