Yeah. I'm not too worried about it. He only has two years remaining after this coming year, so it's not like he will be here until he's in his late 30s or even really much of his mid 30s. Moreover, it's not like Horford is really beating many opponents with his speed and athleticism, so I don't see a reason why his game of hitting jumpers and making deft passes from the top of the key should really deteriorate much.
Worst case, he opens up some luxury tax/cap space when he expires to pay guys like Brown or Tatum. Best case, he's still productive in 3 years and he takes a team friendly extension to win titles in his mid-to-late 30s.