I think your idea of throwing away a prototype is a great one. We essentially did that when we launched our startup, although it was just a major refactoring rather than a complete ditch-and-rebuild effort.
Beyond that, oh man...
Even if the non technical cofounders sit on their asses and do absolutely nothing while you code all day, you'd be sitting on 33% equity in an already successful startup, when they took all the cash risk up front to prove the thing works.
It's this mindset that makes a lot of would-be technical co-founders see non-technical people as a threat. I don't care if you're a technical or non-technical co-founder, if you're not going to pull your weight somehow, I don't want to be in business with you.
Also, to be honest, the "cash risk" is essentially irrelevant to me because I have the skill set to build and validate the MVP myself. There's also a large enough market for my skills that I could sell consulting services (yes, at north of $100/hour) to cover any financial costs to test an MVP.
All you're really bringing to the table is the first 3 steps, and asking your technical co-founder(s) to handle the next 617 or so. It's better than just a "big idea", but not by as much as you're thinking.
All you've got to do now is code and build on existing success. And you think that's an offer that you should scoff at?
Oh, is that all?
Here's a hypothetical scenario: let's say Toyota builds a concept car and takes it to an auto show to demonstrate it to customers. They get fantastic feedback and decide to take the car to market. So, you'd be willing to take the concept car design and find a way to manufacture it in an efficient and cost-effective manner? I mean once the customer validation is done, all that "engineering" stuff must be simple, right?
Metaphor aside, an MVP is vastly different than a truly sustainable product. If it's a true MVP, there will be a myriad of unsolved problems (scalability, maintainability, etc.) that only a skilled engineer can solve. Not to mention, what happens when the market inevitably shifts, rendering some or all of your customer validation effort useless?
The most important aspect of any startup is the dynamic of the team. The most important part of that (in my opinion) is to have clearly delineated roles, roughly equivalent in workload and importance. The best teams have members with complementary and supplementary skills, and everyone respects the skills and contributions of everyone else.
My attitude puzzles you because you're assuming that 90% of the risk comes before the MVP. Launch is very different than break-even, which is very different than sustainability or a liquidity event.
I'm a huge believer in the lean startup (I launched and sold one myself) but you have to recognize that the initial customer validation is just the first step down a very long and arduous road. It's hard enough to walk that road in the first place, let alone if I have to carry you on my back.