That isn't hypothetical, in the above rewrite we took one part that was considered too essential to wait brought it in, wrapped it, and used the new framework. It was working much sooner than the other code. It still isn't completely re-written - but it doesn't need to be as the core business logic is known to work.
https://www.amazon.com/Working-Effectively-Legacy-Michael-Fe...
If you start from scratch you may bump into the same edge cases that the original writers bumped into, and end up with a code that is not much better than the original - even in the original is 2 years out of date.
I’m sure there were cases when writing from scratch was a good call, but I don’t remember hearing about it.
The issue is primarily that of the reward versus cost -- especially the opportunity cost.
When the system is rewritten, will the business have increased revenue or decreased cost? Will it do so significantly, surpassing at least the cost to rewrite (salaries, etc) -- that's the absolute minimum bar, but then you have to consider the opportunity cost which is the real concern:
If you had instead spent the same amount time adding new features, implementing an A/B test suite to increase conversions, improving marketing capabilities, retention mailers, or really any other activity that could positively impact the company business metrics -- would the impact be better than the impact of the rewrite?
In most cases the customers (internal or external) don't really know or care how good/bad the underlying code is, as long as the product serves their needs. When that's true, even partially, the value of 'rewrite' almost never exceeds the opportunity cost alone, let alone the absolute cost (and that's to say nothing of the risks).