It's an interesting idea on worlds with lower gravity and little-to-no atmosphere, like Mars or the Moon. But frankly the fuel needed to get to Mach 1 on Earth is a pretty small fraction of the fuel needed to get to orbit. Orbital rockets get to mach 1 really early in flight. Maglev-assisted launch has never really been seriously developed because it's simpler to just design a rocket to carry a little more fuel.

Also, a lot of rockets aren't designed to handle horizontal loads. Neither are a lot of satellites with big mirror segments, like spy satellites or space telescopes. They live their lives either vertical on Earth, effectively-vertical while in flight due to acceleration, or in microgravity on deployment into orbit.

There's also the issue of being able to reach multiple orbital inclinations for different purposes. The fuel needed for dogleg maneuovers after launching from a maglev track could be more than needed to simply launch vertical from a conventional pad, and is way cheaper than building launch tracks for all the common inclinations.

Maglev-assisted launch also has the issue of engine start-up failures leading to disaster. A conventional liquid-fueled rocket will usually run for a few seconds while still clamped to the launch stand while the flight computers do automatic diagnostics. Issues on start-up mean an automatic safe shutdown is possible prior to releasing the launch clamps. If there's an engine start-up issue with a rocket late into its maglev launch, failure changes from "unload the propellants and try again tomorrow" with a vertical rocket on a pad to "yeeting an out of control rocket and its valuable payload into whatever is downrange".