The biggest advantage of pre-clearance from any point of view other than DHS is it allows flights to airports without customs. Not sure BRU has that kind of demand to anywhere. But pre-clearance is offered on a basis of DHS wanting to push clearance as far from the boarder as possible when it can, airline usage isn't really important in that decision.
Not true - an airport in the US must still have customs support available. Don't forget the inaugural YVR-SNA several years back - it was already in the air and had to divert because SNA didn't have any customs facility.
It doesn't need to be a full FIS (see LGA and DCA as key examples) but if for some reason, US CBP needs to meet that cleared flight, the airport has to have them on-site.
As for pre-clearance, it only makes sense from select Canada markets where there's enough volume. As a US citizen, I hate pre-clearance. I have Global Entry and I think the longest I've had to wait in the past two years pre-COVID (which equated to about 80 entries into the United States) was maybe 3-5 minutes a handful of times in ATL. I'd rather not have to get to the airport so early for pre-clearance, especially since all the times I've done it that I can recall (I've pre-cleared in YYZ, AUA, NAS and DUB before) - all but YYZ required going through normal security screening, and then another screening (to full TSA standards)...it's a pain in the behind, especially if you're terminating at your destination...it's much wasted time.
I'd be more than happy if the US government let preclearance be in place at YUL, YYZ, YYC, YVR and maybe YEG/YHZ/YOW (harder to justify those in my mind though) and get rid of the ones everywhere else.
I should have said FIS,as you are correct, but most airports have access to US CDP personnel for one-off flights, we even do in small CRW. From an airline perspective I more or less agree with your last paragraph, but for DHS the point is to screen foreigners outside the US when possible, the airline business side isn't really important to them.
717, 733, 734, 738, 739, 744, 752, 763, 772, 77W, 789, A319, A320, A321, A332, A333, A359, MD88, CRJ, CR7, CR9, DH1, DH2, DH3, S340, ER4, E170, E175, E190/CO, NW, US, AC, NH, AA, UA, DL, WN, WS, SK, VY, LA, QF, AR, AV, MH, KA, AS