In my mind, better the devil you know. Iran has negative intentions towards us, but not the capability to really do much about it. It's also actively engaged in Iraq, Syria and Yemen - whilst being threatened by the USA and Israel. Meanwhile we're successfully dismantling terror networks, fencing the border with Iran and Afghanistan, racing ahead with Gwadar and other CPEC projects, raising additional security for the project and looking to take off economically.
Why would we risk that for a conflict? We know for a fact the Americans aren't able to do regime change. They've not managed it in Iraq, Syria, Afghanistan or Libya - in each intervention they've created chaos and then abandoned everything other than oil fields. In each country they intervened in the countries are controlled by people who hate the Americans.
If we end up with that scenario on our border with Iran, we risk the success of CPEC, we risk another TTP like insurgency (through some shia elements who are more loyal to the Ayatollah than to our state), we provide India and America an opportunity to fund more violence inside Pakistan (The Indians will do it to weaken us, the Americans will do it to create a roadblock to OBOR).
F--k both sides, let them bomb the shit out of each other, let both of them go home claiming to be champions. As long as it's limited and doesn't create a load of chaos, it shouldn't bother us. It's the people of Iran I feel sorry for. They might be arrogant gits (never met a Persian who wasn't) but they're an intelligent people and are hamstrung by the hatred of America and the Ayatollah. Iran would have been a first world country by now had it not been for sanctions and Mullahs.