Repeating myself again.
If you renegetotiate and extend IT this summer, he'd start at the max salary for a 0-6 player, currently forecast at $25.25 million. His 2018-2019 salary would be $27.3 million. If he is re-signed as a max free agent instead, his salary would be at the 7-9 max, currently forecast to be $30.9 million. Over the same 4-year period (2018-2022), the max this summer would cost $121.2 million, while the max next summer would cost $138.4 million. In the final year, he'd make $33.3 million of renegotiated this summer, and $38.3 million if signed in 2018. Despite these future savings, he'd make more over the next 5 seasons if renegotiated this summer, $146 to $144 million, since his salary next year is so low and he'd be getting an early raise. So he'd absolutely accept the deal unless he really wanted to test free agency.
If there is no max free agent, and the math has always been very difficult for that if we got Fultz, this is the best path forward for our cap space (it does not preclude any kind of trade). I find it unlikely that IT would accept anything less than 5 years, and his play in the last two years has earned a full deal. Doing it this way does make it a bit safer, as he'd be making around 86% of his max in the final year.