I already wrote in this thread that I would have nothing against building churches in KSA provide that 1) we had retained a significant native Christian community (which despite Christianity reaching modern-day KSA shortly after the death of Prophet Isa (AS) this has not been the case since the 10th century AD), 2) that Christians in KSA were not temporary migrants that neither had citizenship nor permanent residency (very rarely).
If somehow the current laws in place will get changed (which points to being the case) and a permanent residency and even the possibility of a "green card" gets established, which also would include non-Muslims (possibly), I would not be against building churches but it should not be in Hijaz preferably (Makkah and Madinah should not even be thought about as candidate cities) and the state should not finance this. Just like we should not finance synagogues, Shinto temples and what not. It would be a different question if, as I wrote, we had a native Christian community. This is no longer the case other than a unknown number of converts.
BTW who is going to fund those churches? I know that a few Western expats (especially those Americans affiliated with missionary work and smaller Christian sects such as Mormons, Baptists etc.) would probably somehow get the money to finance the construction of 100's of churches but what about the average Christian Arab, Ethiopian, Eritrean, Filipino etc. that traveled to KSA for work and to earn more?
Once you open up and allow the likes of the Catholic Church etc. you have opened up a pandora box. Not sure if that is the right way to go currently. Expect increased missionary work, increased political interference, people whose loyalty to the state/its traditions and main religion, can be questioned etc. That would not be a thing if there was a native Christian community as their loyalty would not be questioned as they would have been sons and daughters of the land. Say in Najran.
This is not simply a matter of black and white or yes or no. Not today at least and not under the current circumstances.
BTW before I get accused of being anti-Christian, I have family members who were or remain Roman Catholic.