Just your opinion. Nothing more.
He'd be a huge upgrade over Tyler Zeller.
Not just my opinion - it's the opinion of probably about 90% of all writers who have ever written an article about Kanter, plus the vast majority of the public/fans...and there are statistics to back it up.
It doesn't matter how you judge defense - statistics or subjective 'eye test' - Kanter is a horrible defensive player by any and every measure.
As for him being a huge upgrade over Zeller...maybe on offense, but defense he's be a huge downgrade on Zeller (who is already a significant defensive liability as it is).
Finally, when is it that we all decided that being an 'upgrade' over an existing starter immediately justifies paying a person $40,000,000 - $60,000,000?
A max contract is anywhere from 25% - 35% of the cap, meaning a max contract player takes up roughly 1/3 of your entire payroll. This means that as a general rule, you can only ever afford to have up to three max contract players on a roster...and even then you are left with next to nothing left over to add players around those guys.
If you are willing to give a guy a max contract, then he needs to be more than just an 'upgrade' - he needs to be somebody who is good enough to be a top 3 player on a championship contender...any less and it's not worth it. Look at the Heat and Cavs - those guys took the "three star" approach and had Kevin Love and Chris Bosh as their third best players. To be a max guy you don't necessarily need to be Love/Bosh good, but you need to be around there somewhere at least.
This season there is a LITTLE bit more room to move because of the increasing cap (which means a max contract won't be 30% of the cap after a couple of years) but even if/when the cap reaches the proposed $109m, a $13M - $15m contract is still significant.
It will be the equivalent of $8m - $9m now - and you wouldn't just go and throw away $8m - $9m like it's insignificant at today's cap.
I understand the attraction Kanter offers, but he's a guy you need to be careful with. He's a guy who needs a very special set of players around him in order to really excel. He MUST be paired with a good (and agile) defensive PF or you will fail in an epic way. There is no team in the league who fits this better than OKC, with Ibaka. It's a match made in heaven.
So unless you have money to throw away, and you are confident you can get an excellent defensive PF to put next to him, it's just not worth investing that type of money.
Back to Biyombo again
I'm sorry, is this where you try to give me an explanation as to why Biyombo is NOT a good defensive player?
Because that's what the original question (that I replied to) asked.