Dual licensing (using the GPL and a separate proprietary license) is kind of a hack solution that takes advantage of the fact that business hate the GPL. It can introduce some problems (it effectively bars you from accepting contributions unless you use a CLA, which many contributors won't do). However, while community is an important part of Open Source, the most important part of Open Source is the lack of restrictions on how people use/modify/share the code, so while people can debate whether or not dual licensing is a good idea, that doesn't mean the GPL stops applying.

Any code that is GPL licensed is Open Source. It might be distasteful to some people to force contributors to sign a CLA, you might get some criticism from some segments of the community, but it's not problematic in a way that means it's fundamentally non-FOSS.

BSL on the other hand is not Open Source, but becomes Open Source at the point where the BSL license expires and is replaced by an Open version.

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Personally, I might get some pushback on this, but I actually kind of like BSL more than dual licensing. Dual licensing relies on the fact that people find the GPL toxic. It feels much more to me like a temporary solution, and one that only works by kind of dragging the GPL through the mud. Even among people who don't hate the GPL, it encourages them to think of it as a tool to enforce 'fairness', rather than as a complicated way to use copyright to push towards a world where every user has the rights guaranteed in the GPL for every program they run.

TBH, I vaguely suspect that some of the movement towards SSPL is an evolution of people's attitude towards dual licensing, where they thought that the un-attractiveness of the GPL was the point of the GPL, and now feel like it's not living up to it's 'promise'. The fact that Amazon is able to use GPL code to provide commercial services is seen by those people as a bug, not a feature.

Many of the downsides and restrictions around community contributions with BSL are also present in dual licensing because of the implicit CLA requirements in dual licensed projects. So it's not clear to me that BSL is more harmful to community-built software than dual licensing, and given the above trend, it seems a bit more honest (for lack of a better word).

Because dual licensing doesn't really affect companies like Amazon, it kind of encourages people into these arm races where people say that the GPL has failed in its job because some companies don't hate it (again, the point of the GPL is not to be impossible for companies to use). BSL on the other hand is very straightforward, and because it's upfront about its goals, it's not subject to the same kinds of weird arm races and escalations. You release software as proprietary, we all recognize that it's proprietary and that you want compensation for it, and then at some point it becomes Open Source. That's a really simple model to think about and build around.

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But all that being said, code that is licensed under the GPL is Open Source, period, regardless of what other licenses it is simultaneously offered under.

BSL licensed code before it expires is not Open Source or FOSS: it's proprietary code that later is Open Sourced once a certain amount of commercial value has been extracted from it.