This entire case was just one big bluff, Cooper didn't need a watch since there was nothing he could do if they didn't keep his schedule...
Remember: "Miss, all I have to do is touch these two wires ..." ..."no funny stuff or I'll do the job". He presented a contraption that looked like a bomb.
Cooper could have said, 'ok I give up. Land this plane and I'll cooperate'. It's over.' But he didn't.
He relied on psychology, a parachute, whatever skills he had with a parachute, the darkness and weather, enough time in the air to confuse everyone, and it appears he may have ejected close to where it all began at Portland. He was so far ahead with so many unknowns operating, over a large enough geographical area, a hand full of law enforcement couldn't catch up to him easily. Then money turns up on a sand bar of all places, ten years later, and the only thing to connect these events is the flow of water. And whatever evidence they had also conveniently disappears or is so conflicted it is almost meaningless!
Who is bluffing whom?
This may be fanciful, but it was like fate conspired to let him remain a metaphor. A desperate, faithless, resentful Everyman - a middle-aged loner, not a young hothead - who devised the perfect plan designed to get him what he wanted with the least amount of collateral damage. Tosses his necktie on the way out the door - it was only ever a clip-on, anyway; he was never going to let it choke him; his persona was only ever for show. Vanishes, with every evidentiary detail thereafter, including the find at Tina Bar, producing more questions than answers. The most key pieces of evidence going missing some time before the technology that could have nailed him. *poof!* The magic trick is complete.
Been watching Better Call Saul and Barry, both shows about anti-heroes, how they got there, and how life is lived beyond the time of possible redemption. Shows that make the audience question itself in its ability to empathize with questionable people. We all know the long-term psychological effects Cooper's crime had on some, yet no matter what background we imagine for him, we have to wonder how he got there and how he justified it in his mind. The desire to escape is powerful in our culture, and the ability to do so is rare. That's why watching/reading re-creations of Cooper's jump are enthralling - much like the opening scenes in Casablanca, when the young couple looks wistfully up and the woman goes, "Perhaps tomorrow we'll be on that plane." We don't know what Cooper was REALLY escaping. We can only look up and imagine.