People act as if there haven't been franchise-caliber guys drafted since 2011. Truth be told, there have been a handful of guys taken who can be the best player on a le team. The team moved on from Robinson and Duncan. Kawhi is nowhere near that level of foundational piece, even though he is/was one of the very best players in the NBA. There was life after those guys, and there'd be life after Leonard.
Off the top, there's been maybe 5, if I'm being generous, who could potentially be the best player on a le team: Davis, Antetokounmpo, Embiid, Towns, Simmons. In other words, you either need to hope to get the number #1 pick in the right year, with already relatively low odds decreasing next season or you need to hope to strike gold in the mid round, which is obviously even more unlikely.
There would be life after Leonard, but it would officially end even the notion of thinking about contending for a championship unless they struck gold with whatever they got in return.