Harden already has a few endorsements, and he's already one of the most recognizable guys in the NBA (even if it's mainly because of the beard).
And I see no reason why Serge couldn't snag some endorsements here or abroad (thanks to the Spain thing). Not to mention that he's a handsome MF and could absolutely get some $$$ from non-traditional athlete endorsers (fashion brands, etc.).
The key is keeping them from hitting the open market. These guys know the FO has their shit together and will offer them as much as they can and keep winning.
...
Y'all didn't believe me when I said Harden showed signs of greatness, and you don't believe me now.
I'll be back in a few months to laugh in your faces after James and Serge sign extensions.