Next.js is incredibly popular in the industry. Some of the largest companies out there use it. Whether they host on Vercel or not I can't say, but I'm sure that number isn't zero.
Of course we are talking more about auxiliary stuff like landing pages, marketing sites, blogs and documentation, not hosting for their core business.
Vercel's revenue was 25.5 million in 2022 (according to Nathan Latka who has pretty reputable figures) on a 2.5 billion dollar valuation [0]. They're hyped but it doesn't seem like any big players actually use them at scale, as it's likely much, much cheaper to host on bare cloud providers like AWS, which Vercel themselves wrap (with Vercel also now wrapping the services included in OP's site).
He interviews them on condition of revealing the numbers, which he sells to investors. He has enough clout in the industry that people want to reveal their numbers to him in exchange for being able to raise more money.
I know of five or six companies using Next; none of them use Vercel. I tend to think that their moat against “write a five line dockerfile” just isn’t all that large. I use Vercel for my personal site; I’m familiar with the product suite; I genuinely don’t know why anyone would pay the markup beyond not knowing better.
The auto branch deployments on PRs is pretty cool.
Larger companies already have their workflow, compliance etc.
Developer productivity alone is not going to be worth going through audits / signoffs. It doesn't add enough to the product itself e.g. does it beat Cloudflare, Fastly, Akamai, etc in that space? No.
Of course we are talking more about auxiliary stuff like landing pages, marketing sites, blogs and documentation, not hosting for their core business.