I don't see this to be really affecting insurers at all. The ACA does not really require insurer's to do anything that is totally out of the scope of their interests. It is a bit like the EU carbon surcharge for flights - if all carriers have to pay it, then your business might grow a tiny bit slower, but it does not distort the market. The changes that trumpcare is going to bring - no more individual mandate, permission to remove certain coverages like maternity from plans, and replacing the individual mandate with a penalty for coverage gaps - look to me largely like a zero sum game from an insurer’s perspective. The significant change is the medicaid expansion, and the tax laws.
Biggest reason though that this would not affect any stock markets - the current bill is DOA in the Senate. They lost votes on the far right and on the moderate side. I start thinking that this whole thing is just for show. They don’t have a viable plan, they know it, and they rather keep the ACA in place as a devil’s effigy. Just gotta make it look like they tried.