I'm inclined to believe SSE will be done before EPCOT's 50th or will be much much later. While I could see them phasing out lining up ride releases with anniversaries, having the park's mission statement closed during the 50th would be quite sad.
Imagine Cinderella's Castle being covered in scaffolding during WDW's 50th Anniversary...
Figment I could see being 2030 if they hold off on SSE or 2032 if they focus on SSE first? If we're lucky maybe both beforehand, but something tells me that's not super likely