What if your product is super abstract? What if it requires research and new types of engineering?
Maybe this fits for some types of businesses, but I suspect it's probably better just to build the thing.
Edit: take my opinions with a grain of salt. I've never had a successful exit. I'm mostly wanting to gather others' opinions.
9 out of 10 wantepreneurs that approach me with their SaaS/app idea can validate the offering by building a web site. Instead, everyone wants me to spend months on building a prototype. Which isn't even a prototype, they want full-blown product. "How am I going to sell it when there's no product?!". Dude, here's a landing page with ALL the features YOU THINK people want listed on it. And here's a "Buy now!" button. If you can't get a decent conversion with that, I'm not going to build your app ever. And guess what? Most of the time nobody clicks on the button, because nobody wants that product.
This is even more reason to do market research before sinking a bunch of cost into R&D!
The two cases where you can spend big on R&D without much in the way of market validation are:
1. The product is obviously useful, e.g. self-driving trucks
2. You are going for a patent / aqui-hire play
Point (1) is particularly dangerous when you get attached to your product idea.
Some products need to be tangible for a user to perceive the benefit. Likely everyone can name at least one product that sounds stupid but is actually great. You need to _experience_ them to _get it_.
In the end this is still all product-market fit. In most cases communicating the solution is enough. In rare cases demonstrating it is necessary. But it's likely good advice for your average tinkerer who believes their idea is the next big thing.