Ali was not a great technician and his skills heavily relied on his athletism . Without his athletism to fall back on Ali's technical deficiencies became apparent. There is a big difference between the myth and the reality when it comes to Ali.
You're confusing Ali with Tyson. Ali remained on top long after his physical gifts started to decline, due to his ability to adapt. Quite different from Tyson running in to the same punches over and over again against Douglas, for example.