I think we're arguing at cross-purposes, to an extent. The way I see it, there are two issues here...
1. Should we get rid of Sonny for financial reasons? I say no. He probably gets us enough revenue (both actual and as marketing equivalent) to cover his wages. Based on that, I say there's room for him in the squad as an elder statesman to play mostly as a sub or in easier matches.
2. Has Sonny lost it? This is a difficult one. He clearly hasn't been the Son of old. His game last season lacked spark, pace over 10 yards, and the world class shooting we've been used to. But is that due to physical decline, knackeredness, or Ange's system? I think all three -- the physical decline can't be helped, but the other two can be.
In the second half of the Europa League final, our team as a whole looked knackered. Not just because of all the running that comes from not having possession or control of one game, but due to repeating that all season -- so many sprints in their legs. On the rare occasions that we made it out of our half, there was just nothing left in the tank to mount a decent attack. Son was the epitome of that. Or its nadir, rather. I really think (or at least hope) that a refreshed Sonny, coming on as a sub, will be able to make space again, and have energy to get the shot off.
Regardless, I agree that we owe Son enough to respect his wishes -- be it staying on with reduced playing time, heading off for one last payday, or being released so he can play at a club at a lower standard. Even if that means keeping him this season then letting him go on a free.
Some will argue that there's no room for sentimentality in a club that wants to be the best. But every case is different. Son has been a legend for us, pivotal to the best spell we've had since the Premier League began. I see him in a similar bracket to Thomas Müller at Bayern -- a symbol of the club, in a way that few others can be. Based on last season's form, he doesn't merit a starting spot, but I don't think we have enough depth up front to push him off the bench.