Over the past 4 seasons, he has had declines in FG%, Reb, and Pts. Not a dramatic drop, but a trend down. I don't expect him to fall off a cliff or anything like that. He should be a productive veteran player for at least 4 more seasons. We'll see.
I like the signing. He is both a good player who will help us today (at a position of need) but also an asset we were able to add without giving up anything. We are both a better team and also have more assets to use to trade or attract FAs because of the signing. But he is a veteran who is declining (KG was declining too). You can quibble about how much he is declining or how good he will be in 3 years or 4 years but I don't think it can be disputed that he is going to decline.
The problem is that his window and the rest of the team's window aren't really aligned like KG's window was aligned with Pierce's and Allen's. This lack of alignment is not something that would have stopped me from signing him, it is just part of the deal with Horford. It is the downside to the signing. This lack of alignment may lead to him being traded at some point, maybe not. If he is traded, I am sure it will be for some good but perhaps younger players/assets.