Casper is an interesting example to me because I've learned not to buy durable goods from companies that aren't solvent. If Casper went belly up next year, there's nobody providing any warranty for your mattress. And if it's discovered that the mattress is dangerous to sleep on because of contamination during the cheap manufacturing process, good luck getting anyone to recall it and replace it with something safe.
I'd worry less about a product like Casper's than a lot of other products that you expect will need repair/supplies/etc. at some point. Sure it's possible that some gross defect will be discovered in Casper mattresses in a couple years. But it's not like there isn't lots of other furniture that turns out to not be well made. And good luck getting refunds on that after 2 or 3 years in many/most cases.
