If all stars rotate, why was there a theory developed that requires non-rotating stars?
According to Penrose's research, a non-rotating star would end up, after gravitational collapse, as a perfectly spherical black hole. However, every star in the universe has some kind of angular momentum.
Why even bother doing that research if that won't ever happen in the universe and does it have any implications for the future of astrophysics?
Would you mind providing more information about the research, e.g. linking to a paper about it?
I suppose it's the solution to a simplified model of reality as a first step? That's not unusual in science...
FWIW, there's a graph at the end of this answer of the spin of 19 supermassive black holes. As you can see, they have spin speeds that are a significant fraction of the speed of light.
Ever solved a problem by treating macroscopic objects as point masses? None of those objects are actually mathematical points, and yet, you can make certain assumptions, disregard certain details, and use the resulting model to understand what is going on, and make predictions - as long as you keep in mind that there are scenarios where such a model is less applicable, or not applicable, because the assumptions you made no longer apply and the details you disregarded start to produce significant effects. There's always a domain of applicability. Everything in science is like that.
Another consideration is that the physics that describe a rotating black hole was much harder to develop.
I wouldn't fancy waiting 47 years for a more accurate black hole model to be developed before doing any meaningful work in the field.