Well we don't know that yet. Maybe in another 50 years. So far its still popular. Also even though I am not a fan of NuTrek it has help to get the youngins to at least check out TOS. By the time the show becomes irrelevant we will all be dead so none of us will care.
I agree, but "popular" is a relative term. Compared to other 1960s shows, TOS is phenomenally popular; but in the overall media world it's a tiny blip. Whether that will be enough to justify future investment into "re-vitalizing" the property is open for question. There are elements other than picture quality and visual effects that date the show, too: sets, pacing, dialogue style, acting style, cultural aspects... The further away we get from the original viewing era the harder it will be for new mainstream viewers to get into the show. It's appeal will increasingly be to a small, specialized audience who appreciates the historic value or likes the aesthetics of old productions. Those viewers, I think, will care less and less about what some effects team thought should be added to the series 40 years after the fact.
How long the show will remain "relevant," or if it still is, I have no idea and would probably not be a good judge.