I honestly don't know if better arguments are needed. I think most people simply have no idea that this research is happening right now, and if they did the vast majority would be against it. Feels fundamentally different from stem cell or IVF.

We know that human brain cells in sufficiently large and complex assemblages are sentient (I don't know if the word "inherently" has any purpose here). We know this research aims at making the assemblages larger and more complex. It's a fundamentally different kind of argument IMO compared to those against e.g. stem cells or IVF.

(Slight aside but did you know that they're also doing tests on whole human brains taken from corpses, and they give them tranquilizers "just to be sure"? https://news.ycombinator.com/item?id=48212992)