Huh - I was pretty sure (and from experience and observation) thought it was the other way around. (Doing more research leads to the answer: rewrite vs. refactor -- it depends on how bad things are). In the case of Kerbal, a rewrite is basically mandatory because the original structure is limited. But yes, now that you describe it, I agree there will be far more bugs in KSP v2.0.
I am curious about the numerics; I get why they were a problem for the original team. (Going from a rocket simulation to a solar-system simulation jumps numerical requirements by quite a bit). But if a team knows what they're getting into, the numerical implementation should be relatively straightforward. Astrodynamic and high-precision trajectory simulations are topics of active research and they should be able to copy the numerical stuff from them.