Pakistan would develop a fine team in the mid to late 90 after Imran had retired. They would intersperse this with periods of chronic incompetence and then develop fine teams as time went along. That seems to be just the way it rolls and would have probably happened with or without him.
This ignores the fact that Imran was a team builder. He had a direct hand in building that 90s team he left behind.
Let's see.
There is no way Pakistan develop a Wasim, Waqar or Shoaib who were directly inspired by him to become fast bowlers. In Waqar's case, Imran selected him himself while watching him on TV.
Abdul Qadir, perhaps Pakistan's greatest spinner, was in the wilderness when Imran became captain and he fought the selectors to have him included. Same with Inzi. Mushtaq Ahmed was groomed by Imran to be an attacking leggie like Qadir.
Pakistan's decade as captain saw Pakistan win in England, India and the World Cup, as well as compete against the WI. Highly unlikely Pakistan would ever achieve that without Imran, especially without fast bowlers.
What is the reality? Pakistan under Imran became a top tier team by the mid-80s and they stayed like that for much of the 90s after he left, though inconsistency kept them behind SA and Australia as the decade went but still generally more competitive than the rest while playing below their potential.
Post 2003 with the retirements of Anwar, Waqar and Wasim and with Inzi as captain, you can say Pakistan largely retreated from Imran's s legacy of his aggressive brand of instinctive paternalistic cricket and were and have been mid-tier since, with the brief exception for Misbah in mid-2010s which lasted like a year before going back to mid tier.
Conclusion: Without Imran, Pakistan would always be mid-tier, jostling with England, India, SL and NZ for the 80s and 90s without fast bowlers to take them to a higher rung. They wouldn't enjoy that brief 15/20 year period of flirtation at being a top side. They would likely have sooner adopted a Misbah model of defensive home based cricket using local spinners to stay competitive at home while being mediocre abroad.
But I argue that Imran's influence and legacy has mostly gone since the 2000s except for somewhat of an impact on getting new fast bowling talent who lack Imran's grooming to have full careers.