I am sure that Alex is Sam when older. He skates, he reckons the skate was always his, the itchy ear and the way to scratch it, the memory leak already affecting right outside the Pasage, the explanation on how Alex is able to paint Noah and Emma in the mural, the interest shown in being rescued by keeping and placing the phones right where they would be found to trigger the search. Yes. Not a theory. Not an ambiguous ending but a great, obvious one. If they make a season 2, I’m sure they will develop this, and show how Sam/Alex has travelled through time, and how he got the funds to build the resort and get his goal: meeting Hannah. They will probably introduce us to an older Hannah and show how Sam/Alex has traveled through time far into the past as well (meteorites… dinosaurs…).