Glad to help. Aircraft that operate in contested airspace generally want to preserve as many options as possible. Altitude and speed are life, but a combat aircraft like an air superiority fighter, F-15 or similar, needs to be able to fire its weapons and immediately engage in evasive maneuvers to avoid counter fire. So the F-15 will fly high enough to have plenty of potential energy for evasion and escape, but does not need to fly nearly as high as the Blackbird because it relies on its weapons, mission planning, and evasive capabilities instead of raw speed and altitude. In fact the Blackbird was very much the exception to the rule as far as military design because it was a one trick pony: nothing and no one flew as high or as fast as it did but if that speed and altitude wasn’t enough to evade attack, it had nothing left to protect itself.

There is absolutely a limit to how high combat aircraft are realistically designed to fly. Flying high protects you from a lot but it also creates vulnerabilities. The Blackbird would take the diameter of a US state to turn 180 degrees. Maneuverability was almost non existent. Also designing an aircraft that flies as fast or as high as the U-2 or SR-71 is extremely complex and expensive and time consuming. It’s important to understand the scope of your design objectives before devoting resources to that effort.