So what if he get 3y 35 million next year?
You have to weigh the risk/reward.
No matter what happens, Morris' ability to get longer-duration contracts will diminish as he ages. That's just the nature of the NBA (Chris Paul being a notable exception

)
What's the probability he suffers a career-altering injury this year? Or underperforms relative to last year's production? Or walks into an FA environment this coming summer with less cap space than he expected (in part because of all the large, multi-year deals that got signed over the past couple of weeks)?
From my perspective, taking 3 for $41 million from the Clippers to play on a contender makes a whole lot more sense than taking 1 year at $15 million on the Knicks - even with the
potential for a 3-year, $35 million contract elsewhere after that year.
The Clippers would have his Bird rights because of the length of that contract - meaning that even as a capped out championship-contending (or winning) team, they'd be able to bring him back irrespective of the salary cap. They'd just need to be willing to pay the luxury tax (and Steve Ballmer, worth $50+ billion, is willing to pay whatever it takes to win a championship).
That's a huge strategic advantage for Morris as far as getting a decent 2nd contract from the Clippers - if he's still a serviceable player at the end of those 3 years, it makes sense for the Clippers to
overpay him relative to the market because of his Bird rights - he essentially becomes a human trade exception that can be put to work in transactions to add pieces around the Kawhi / Paul core (this is exactly what the Clippers did with the 4 years / $28 million they just gave Zubac).
Only time will tell - but with the facts as they were on June 30th at 6pm, turning down the Clippers' offer was an enormous mistake.