I think All-Star is the wrong measure to use here, because several guys can be All-Stars as the #1 option (Carmelo Anthony for example) but completely ineffective as a #3 option. Al's never going to be a #1 option again (if he ever was)...
But can Al Horford be the 3rd best (or most important) player on a championship contender? I think the answer is yes, and this isn't just talking about a unique scenario where you have 2 dominate players (Jordan/Pippen, Shaq/Kobe, etc.) and any bum can be #3. I think if you look around the league, Al could be #3 best/important (and occasionally slide effectively up to #2 or even down to #4) on a lot of teams hoping to contend: MIL, PHI, BOS, BRK (healthy), GS (ex Klay), LAL, LAC, HOU, UTA, POR, DEN (swap with Milsap), etc.
So as a championship contender, are you comfortable paying your #3 guy $30m? I think I am (if I'm willing to pay that "championship tax" to get there with a big payroll, which you usually have to pay to contend).
I can see him continually being more important to Philly than Harris, and even more important than Simmons (at times) or being able to hold the fort down if Embiid goes down. He just fits in so well, doesn't need the ball, plays great D, can run the offense, attack the rim, stretch the floor, all without needing to get stats.
I think he can play the #3 role very well for the next 2 years. Maybe even the next 3-4. Even in Year 3-4, if things go to hell in Philly and they aren't contending, I think he'll have a tradeable contract (like Marc Gasol this year) with several contenders willing to take on his contract, because he can fit so well with anybody (and this is the opposite of what we have with Westbrook/Chris Paul where there's big questions of fit and maybe only 1 team is crazy enough to make that gamble if you're lucky).