The system may return, but less diversity means less resiliency. The 'return' isn't a return to baseline. That system return is an attempt to attain a state of function that is no longer available. That leaves this returned system in a constant state of stress. Stress on any system reduces function, increases failure rates, and extends repair times greatly. When a stressed system attempts to heal, many factors inhibit or outright prevent healing from occuring.

That's all observation, reasoning, and logic. Merely observing what occurs in a system that loses or gains is enough data to make fairly striking conclusions. The systems never return to baseline. Not once. Not ever. We've never observed a return to baseline after a species is destroyed, nor if an invasive species enters the system.

Once balance is lost, it cannot be attained again. Will not be. Has not been. Observed data is clear about this pattern. There is no equilibrium. There is a system that is functioning and then there is a system that no longer functions. There are no return to function systems in nature. Humans have not observed a mechanism that would bring that about, nor observed the occurrence of such.

One failure leads to another, and to another. Cascading. That's what we observe over generations of data collection.