I'd happily sign Horford to a max contract, there isn't a better way we could conceivably use our cap space.
There are at least a few ways, aren't there?
1) A trade for a better, younger player than Horford without having to send out any contracts attached to good players from our end.
2) Saving it for a better free agent in 2017 when the free agent class will be deeper and better, potentially including the best free agent from this upcoming class if Durant opts for a 1 + 1 contract this summer.
3) Not having an inferior version of Horford for nearly 30 million a year in 2019 and 2020 taking up an enormous percentage of our cap space when the exploded cap will have retreated back to semi-normalcy.