I do think lumping Tim in with Manu and Tony to call them all "Hall-of-Famers" does sell Duncan short. He was an order of magnitude better than his co-stars. That said, I actually do believe the Spurs could have adjusted to losing Tim for a season and win a number of games, even in the playoffs. But that is a scenario where Tim tears his ACL in like June, and the team can prepare a whole summer for not having him. If that had happened with Kawhi, you would have seen a different Spurs team than the one you got this year as well. That's even more true in a scenario where Tim would be completely off the books all season, freeing up that money for someone else. Guys like Dave, Webber, Malone and O'Neal were options around that time, and any one of them would have made the Spurs a respectable playoff team. In that same way, I strongly doubt SA goes into the summer looking to reup Mills if they had a potential max slot available. A number of perimeter-oriented stars would have been available for trades or signings, and any of them added to this basic core would have solidified the Spurs as a second-tier playoff team.
However, both of those species of scenarios are fantasies. SA is dealing with a -ton of uncertainty over when or if Kawhi is going to be back. Not only has that prevented them for making an aggressive move to replace him, but it has hurt chemistry with all the waiting and dashed hopes. In a similar situation, I do think the team would have underperformed their potential had Tim been similarly unavailable. There wouldn't be any "serviceable" big to replace Duncan (have people forgotten that PAFTO failed to get a serviceable big to play NEXT to Duncan?). There would have just been a team with high-potential guards in a league where bigs still dominated. I don't know enough about the specific landscapes of the league those years to say exactly how well they'd do, but it probably wouldn't be an obvious 50 games. Tony and Manu are HoFers, but Tim was a GOAT candidate. It's disingenuous to put them on his level, and it's damned disrespectful to put Kawhi there by comparing the scenarios.