The lesson drawn, and a good one as an investor, was the YC advice that start-ups should build stuff for other start-ups.
It sounds good to perpetuate the "but Amazon.com had extra capacity they could sell" but it's not true.
With that definition, AWS is a startup. So was the iPhone. Both invented their respective markets, and had to grow quickly to do it. If either of them had needed to raise money from VCs, smart VCs would’ve invested. But they didn’t need to raise capital, since they were already a successful company.
But, when the gold rush is over, the shovel-seller may have to do big layoffs.