In general, what is the highest frequency that can be carried over a wire?
I know it can do these resolutions in practice because I have personally operated CRTs at 4000x3000 resolution in the early 2000s. This could be considered "the 4:3 of 4K." It was not done on fancy equipment or high-end monitors. Analog stuff really could just go to really high resolutions and refresh rates with above-average, but typical stuff.
CRTs simply respond to waveforms for red, green, blue, vertical sync, and horizonal sync. That's it. If you want more horizonal pixels, make your scan lines denser. If you want more vertical pixels, add more scan lines. Want a faster refresh rate? Simply run all the signals faster.
There is no hard upper limit to it. With digital signals, there are throughput limits per spec due to bit rates, but with analog, there are no bits. Resolutions like 40k x 30k are theoretically possible. The difficult parts are rendering the signal at these high frequencies, and being able to meaningfully display them. The VGA connection itself has no limits.