That sounds like hiding behind fine print, which is certainly legal, but I'm not sure how that makes it the "right thing". Why isn't the "right thing" honoring the original arrangement with existing customers, at least until it expires?
Sure I understand that Instacart isn't a charity, and they must have realized that this was clearly not a profitable business model. I guess what I don't understand is why they can't own the mistake and eat the loss. They didn't advertise "free deliveries for $99 as long as our math and assumptions hold", they advertised "free deliveries for $99 for a year".
And people paid them money for it.
Why is it unreasonable for Instacart Express customers to expect what they paid for? Why is it acceptable for Instacart to effectively bait and switch their early adopters, regardless of their motivations?