Some people who understood fighter jets felt that the design had reached the very top, that it had peaked and could not be improved upon.
In terms of fighter jet design, one could say there was no room for progress; to enhance the performance of fighter jets further, what was needed wasn't design but equipment, technology from other areas.
For instance, advanced radar systems, stealth capabilities, electronic signal interference for combat, weapon mounts, and so on.
These were not issues that could be resolved through design alone.