(Netlify engineer here) To echo what the others have said, lambdas are a bit smarter than that. They actually keep the node process running until it has been idle for around 5-15 minutes. We can make use of that ourselves by reusing the Next server too. We even cache data locally in the lambda tmpdir (though we do that more for Gatsby). The cold start issue though is one of the reasons that using isolates rather than Node-based functions is so great. Both Cloudflare Workers (used by Vercel) and Deno Deploy (used by Netlify) have extremely fast starts, even when cold.
It's generally just a better fit for this model, which is why I'm so glad to see the approach becoming more popular and am genuinely please that Vercel's offering has gone GA. The more this model of edge computing spreads, the better. More framworks will add support, and it will be a virtuous cycle. The WinterCG work on creating a common standard for these runtimes is also a great project.