I think the debate re. selling Sissoko is being influenced by how divisive he was as a player last season - some loved him, some hated him and I think that's at the forefront of people's mind's right now in this thread.
I think it's right to keep Sissoko and use him as an experienced, stable member of the squad next season. It's the first one in our new stadium, and slipping back would be terrible. At the same time, I don't think it's a simple decision. Here's my reasoning:
For me, in order to recoup the highest value for our players and ensure that the squad continues to develop, those thoughts aren't so helpful. He's the wrong side of 30, right? His game is based around physical dominance (along with disciplined positioning and great teamplay, don't hate me). I imagine that even his most stalwart fans (of which I am one) can admit that. If we take it as a given that his capabilities will decrease in the near future, I think selling him now really has to be given serious consideration (even though he just took us singlehandedly to the CL final).
Unless he has another mega season, it's unlikely his value will be any higher than it is now. Any decent scouting will note his physicality and how important it is. The value of a 31 year old Sissoko is far less than a 30 year old Sissoko. It will depreciate even further if he has a season at 5th or 6th choice, out of the limelight. Ndombele's signing will limit Sissoko's time on the pitch. I think it's fair to assume that there is a high financial cost to keeping Sissoko this season.
At the same time as holding that belief, overall, I don't think it's right to sell Sissoko this season. He's been around a good while now, he has a great meteorology, and he does what he's told. I think that there is a risk of changing too much at once, and we know Eriksen is likely to be off. I don't want to lose too much of our CL final team in one go. I think that on balance, the financial costs to keeping Sissoko this are outweighed by the benefits to team stability (we all know what he's gonna produce!) in the context of Eriksen's departure. If we only thought about the impact on the playing staff and football activity of the club, I'd want to keep Eriksen over Sissoko... but as the financial impact of keeping Eriksen this season is so much higher than keeping Sissoko (80m vs 10-15m) my decision is kinda made for me. Also, Sissoko sparks joy about as often if not more so than Eriksen does for me (controversial, don't hurt me) and I'd love to watch him for one more season in his prime.