It's not a matter of player contributions, it comes down to philosophy. Danny is not going to match what other teams will be offering IT.
I am not sure what gives you such certainty.
By the summer of 2018, it may very well be that the Celts have no better option for their cap space than to re-sign Isaiah to a 3 or 4 year max deal.
The Celts will likely be in a position at that point in time where even letting Isaiah and Avery walk won't leave enough cap space to make a major move for anybody else. The only reason to not sign Isaiah would be if they already have a stud guard (basically, a guy they draft with the 2017 BRK pick) waiting in the wings.
I could see Isaiah leaving town if the Celts balk at giving him 5 years. Maybe. But it really seems in the interest of both sides to keep him around.
I will be uneasy about Isaiah's aging process if the Celts indeed sign him to a deal that will take him past age 31. Still, when you've got the core of a team that can win 45-55 games year on year, it doesn't make sense to just let that walk out the door.