If iPhone 5s can last FOUR years with only 1 GB RAM, the X should last just as long as that. By 2019-2020, a hint of lag and faster battery drain will be inevitable though.
Gotta have that planned obsolescence to keep you coming back for more and stimulate the economy. Then dump all that e-waste to third world countries like Ghana where they can melt it into copper and extract tiny specks of gold from the inside.
Consumers want new new new every year or two. By 2020 when the SE X is released, the X design had grown so old being nearly 2.5 years old, some who love it now wouldn't care for it anymore and would want a new design again. Because a new design doesn't add real functionality to a smartphone. It just keeps things fresh to the naked eye.
Smartphones are sold to us like fashion accessories and jewelry. Disposal like old shoes and socks.