There is a category difference between is the site up and can a customer still buy. Almost all website monitoring answers the first question, which is easy to measure and rarely the one that costs you money.
A website is not a document that is either served or not. It is a set of paths people take, and every one of them can break while the server behaves perfectly.
Uptime monitoring measures the response. Real user monitoring measures the people who arrived, so it tells you about a break after enough customers have hit it. Neither is watching the journey itself, continuously, whether or not anyone is on the site.
That is the gap. The failures that cost the most - broken checkout, dead form, unusable booking - all return 200 the entire time.
Most know they should. The reason they do not is cost of maintenance: writing Playwright or Cypress scripts is a day's work per journey, and keeping them passing through every redesign is a permanent tax. Suites like that are abandoned more often than they are finished.
So the honest question is not whether journey testing is valuable. It is whether it can be made cheap enough to survive contact with a real roadmap.
Four steps, none of which involve writing a test script.
Add your site, describe one journey, and let it run. Free, no credit card.