If they trade Kawhi to the Lakers, you think there won't be a massive decline in sales and butts in seats? When it comes to business, trading him to LA and watching him win les as the national media crows about and reinforces the narrative that SA doesn't treat players well ... That is the worst thing they can do.
In this day and age, no one has to be a fan of anyone. Every game from every team in every major sport in the world is accessible in an instant. The only thing that keeps most people fans is an emotional connection.
If the Spurs give Kawhi to the fans most hated rival, and without Duncan, soon to be no Ginobili ... That connection will lessen or sever for so many, the attrition so significant, that the finances of the team may not recover for decades.
The Charlotte Hornets were the NBA's most popular home team, sellouts galore. The toast of the league. But when the fans turned on them due to the owner, that was it. It's been 20 years, and they still aren't back. That's what can happen when the fans collectively move on.