I expect Horford to opt out of his current deal after Year 3 and sign a 4+1 deal in Boston for a good deal less than the max.
Not sure about Hayward. I imagine something similar, although potentially at or near at least the 7-9 max for him.
Horford would play to 36-37 years old in that scenario. I think his game will age very well. But do you think he'll hold up for that long?
Can't say for sure, obviously. But he's generally not been a player who's dealt with a lot of nagging injuries over his career. He's had two issues with his pectoral muscles -- those give me some pause, but he's had three full seasons since the last one, so the best I can do is assume that's in the past at this point. He's not a guy who seems like he'll be derailed by back/knee/foot issues like some bigs are. And he's not someone who's looked worn down at the end of a season, and has had many years going multiple rounds into the playoffs.
Father Time will slowly erode his abilities, but I wouldn't be worried about the wheels falling off during his next deal.
I'm not too worried about the pectoral muscle surgeries. Pectoral tears are rare in basketball because the activities simply don't put a ton of stress on them like, say, being an OL in the NFL. Al just happened to have a chronic defect issue that, from my understanding, was addressed by the surgeries.
Overall, I'm in the "Al's game should age well" camp. I know that folks who focus on counting stats think that Al's game has declined the last few years, but that's deceptive. What he's done is shift his game quite a bit. Positionally, he plays more and more outside the paint on both ends of the court. This has dropped both his scoring and rebounding numbers, but his increased usage as a playmaker has more than made up for it in assists on one end and his value in switching defense on the other.
Consider that, when you add both points-scored directly plus points created via assist, Al actually had his career best year last year, creating 26.0 total points per game. He did that by scoring 14 and creating 12 via assist.
If Al was more scoring-centric and less of a passer, scoring 20 and creating 6 via assist, everyone here who complains about his max contract would be ecstatic. But he wouldn't necessarily have been any more valuable.
He also shot the most 3PA per game of his career and posted his 2nd best 3PT% with a solid 35.5%. He's basically transformed his game to fit the modern NBA.