Maybe you're right that in a few years, I'll be able to buy a 6TB SSD for the $200 I'd pay for a hard disc today, but speaking as a user about to buy a computer

Maybe you're right that in a few years, I'll be able to buy a 6TB SSD for the $200 I'd pay for a hard disc today, but speaking as a user about to buy a computer, I want one that works great today. Which means worrying less about filling up my paltry 128GB SSD. The other, non-technical, solution is for Apple to make 512GB SSDs standard :) But Apple is going in the other direction, introducing a 128GB SSD.