His nonexistent backfoot play was always going to be a massive issue once he got older and his eyesight and reflexes declined. That's exactly what happened. Teams figured out as long as you bowled back of a length to him outside off he couldn't score. So he became overly reliant on a risky shot (cover drive) to score the majority of his runs. That just won't work on most wickets that aren't flat.
Looking back it's almost astonishing he had the peak that he did with such a limited arsenal of shots. Youth and dogged determination sure goes a long way.