So here is the evidence: We know the big bang fulfilled the math criteria of being a black hole. We also know how black holes work from the outside from observations today. The simplest theory explaining both of those is that the math for the inside of the black hole is wrong, while it is right for the outside, and that we observe in our own universe/black hole is what really happens inside. Any other explanation simply has less evidence behind it.

Speculating that there is some strange singularity there is just as much sci fi as speculating that there isn't and that what we observe from the big bang is the normal state. Any theory saying that the black hole has a singularity inside would need to explain why the universe isn't a singularity, otherwise its provably wrong, since we know the universe isn't a singularity.

The only evidence against the universe and the big bang being a black hole is that it lacks a singularity, everything else adds up to it being a black hole. Ignoring that evidence is more unscientific imo. Also saying that the universe expands due to "dark eneregy" is also baseless sci-fi in that case.

Anyway, if black holes do become big bangs like that then it could be testable by looking at distributions after a big bang etc and compare that with some different ways of calculating black holes sucking up mass in different scenarios to see if it is reasonable. Science starts with speculating about stuff, speculation is not non-science. At least this is more testable than stuff like dark energy.

Edit: I've also studied all the math and physics in college for the testable parts of GR, I haven't studied string theory, at that point I no longer felt the physics made sense, I don't think it is wrong for me to hold alternative theories to the untested ones.