He's worth it, absolutely.
The question is, will we give it to him? I'd say probably, but Danny will likely try to finagle some concessions or salary flexibility. I don't think we're enthused about paying him ~30 mill as he crosses over 30, but I don't see many other options; we're not likely to let him go elsewhere unless there's a major injury, he's much too popular.
He's much too popular
and replacing him with an above average starting point guard, let alone a player of comparable value, will almost certainly not be an option, and even if it is, it will likely cost about the same amount of money with a similar level of long term risk.
Even if you could find a replacement -- let's say you trade for a guy like Goran Dragic, who makes under $20 million and provides a significant portion of Isaiah's production -- that won't get you closer to a championship unless you use the money you save to realize a net improvement in the talent level of the team.
If you get a guy like Dragic, perahps you can keep the team moving forward with a similar albeit lesser type of player at point guard. But your chances of doing anything in the playoffs are much less, because marginal value at key positions is what gives teams an edge in the post-season. Or in other words, talent wins. You don't get extra points in the playoffs for filling your roster with bargain contracts.
So perhaps you save $15 million on the cap sheet, but the team will be over the cap in any case, and even if they weren't, you probably won't be able to spend that $15 million on a player who will more than make up for the loss of talent at the lead guard spot.
I suppose people may be thinking that we'll draft our next franchise guard this summer, which will allow the team to move on from Isaiah. Even if Fultz or Ball is the pick and he turns into a superstar, that doesn't mean the team will have the opportunity to use the money that it saves by letting Isaiah go in free agency on a player at another position with a similar level of talent and productivity.
All of the above is why when you have a superstar player -- which IT absolutely is -- you don't let him go. You do everything you can to keep him and then find complementary players to put around him.