While I greatly enjoyed John Wick, and have been a fan of Keanu since Bill and Ted's Excellent Adventure, I wish they had not left it ambiguous (even if only 10%). It would have been a stronger ending than leaving it a little open.
On top of that, I think the best way to make sure a story is well liked in the future is to tell the story you want and end on a high note. Unfortunately, Hollywood is very interested in franchises and would be remiss if it allowed a potentially milkable franchise to end. Not leaving it open 10% would have prevented a sequel that tarnishes the rest of the franchise.
Finally, one of the reasons I think everyone was drawn to the original John Wick was its simplicity in story. John's motivation was simple, and the story wasn't bogged down by movies of lore/ canon. I would rather Keanu/ Stahelski come together on an entirely new project than make a John Wick 5.