In case ya'll think I'm just biased, take read through the comments, there are some great ones (WSJ has some of the highest quality comments online due to their paywall).
I think the article has a few flaws.
A) The dot com bubble is famous not only for the massive funding, but also the extreme valuations in the tens of billions across hundreds/thousands of companies. Jet is a calculated one time bet by private investors in an extremely different environment.
B) Success breeds success. Marc Lore already launched a successful website, no one from the dot com had that pedigree.
C) Probably the only way to launch a massively successful ecommerce company nowadays is to sink a lot of money into it. The losses will slow as they add merchants. Subsidizing losses is a common practice that some ecommerce companies are doing on a much larger scale than Jet. Including eBay, and Amazon. Amazon shareholders have literally subsidized billions of dollars in Amazon losses, and no one blinks an eye. People believe that Amazon will one day pay off, and so does jets investors.
In Jets case it's private investors who at least understand the risk proposition.
D) About 20% of items in my orders are fulfilled by concierge, the article makes it sound like 50%+.
I wonder what the real numbers are. 50% isn't sustainable for too long. But if it's 20% and they whittle it down to 5-10% in a few months then the losses wouldn't be bad at all.