I was doing a similar breakdown back when I bought my System76. The difference was upgradability. If I ever thought I might need more RAM I’d have to buy that up front on the MacBook air, putting its price over 1,700 off the shelf for the max ram. System76 cost close to the base MacBook air model, but I can add RAM and upgrades at my choosing, find the best price, and install them myself when I need them. That was worth it for me.