I can't help but laugh at the fact a thread about Horford devolved so quickly into a debate about Kyrie. Talk about off-topic.
Anyways, bringing the discussion back to Horford, I think there is a chance he could have his number retired here. If Horford plays here another five seasons, which would take his through his age 36 season, that would give him 7 years in Boston.
Horford has already helped guide the team to 2 ECF appearances and the best regular season record his first year here. If he stays another five years and the team wins a couple of titles in that time frame, and Horford makes one or two more all-Star appearances, he'll have a decent case for having his number retired. Also factoring into that equation will be the fact he was the first major free agent to sign with Boston coming from another team.
I'm not sure what percentage I'd put on him achieving enough benchmarks to get his number retired here, but it is certainly possible.