OP if you're talking about me (I had recurrent mcs as mentioned above), I'm pregnant now and it's all going well. I don't have any chromosomal issues and don't need PSG.
I'm 30 now and my mcs were in my 20s. The likelihood of having 3 "random" chromosomal problems causing mcs at that age is tiny, which is why the NHS test you after 3.
I don't have any of the other known issues that can cause mcs, like sticky blood. The only issue that was found and treated was high NK cells, and it worked.
TBH I don't know and I don't think they know precisely how some of the immune treatment works but I know that in my case and in many others, it is prednisolone that has enabled me to keep this pregnancy going. Once my immune system was suppressed with the steroids I stopped miscarrying.
I have other autoimmune conditions though where my body attacks healthy cells (as do many people in my family) so it makes sense to me that my immune system has also attacked developing embryos in the past. There are lots and lots of conditions (thyroid issues, diabetes, arthritis, crohns, lupus, Addison's, uveitis etc etc) where the body's immune system is unbalanced and it attacks itself, so not surprising that there may be other conditions where the embryo is the target, as opposed to, say, the thyroid gland, the pancreas, or the villi in the intestines. Sure the body should "find a balance and work with the developing baby" as you say, but what if it doesn't?
Obviously this won't be the case for everyone and it's not a panacea, but I wouldn't write it off entirely.