Here is why the "one physics simulation should cover it all" idea will never work: The word "physics" is just too broad. Is it about particle simulation (classical N-body simulations)? Or is it about fluid simulation (classical CFD = computational fluid dynamics, or classical Navier-stokes)? Is it about a game engine, digital art or just for a fancy GUI? What about the hardware, should we use GPGPUs, distributed computing or so?

Talking about the output, what is it the user is actually interested in? Some direct simulation outcome, such as trajectories or densities, still images, time series or some derived quantities? Is the user capable of programming in some language, is interface scripting even required? Does she actually prefer a CLI?

There are a lot of different industries with different needs. For certain engineering tasks (such as aviation), there are long standing tools which work fine and people are used to. For hobbyists or the gaming industry, the same applies.

Disclaimer: I am a theoretical/computational physicist. Maybe I am too much thinking about simulating heavy ion collisions or black hole mergers, but given the physical problem you want to solve, there are quite different numerical methods applied. This applies for fundamental concepts: Besides the traditional linear algebra/partial differential equation solving, there are statistical approaches and nowadays artificial intelligence for obtaining a solution to a problem in a shorter time. The ways how to model physical reality are as diverse as the physical problems.