I don't know. The Sixties was when westerns hit their peak on TV, but they were a movie staple long before that. Just looking at John Ford alone, he started making westerns in the silent era and made many of his most famous westerns in the 30s, 40s, and 50s: Stagecoach, My Darling Clementine, Rio Grande, The Searchers, etc. (Yes, I watch way too much TCM.)
Westerns were no passing fad. Here's hoping comic-book movies prove equally durable.
Also consider that Westerns have an inherently narrow focus since they're all basically fictionalising a set time period and location aesthetic.
Comic books are an entire medium unto themselves and can tell every kind of story imaginable, from 'Road to Perdition' to 'Ghost World', 'The Walking Dead' and 'Valérian and Laureline: Empire of a Thousand Planets'. That is a creative resource several orders of magnitude richer and deeper than the Old West.
So if that genre managed to stick around so long, by rights comic books movies could potentially go the better part of a century...or be back to obscurity by the end of the next decade right before free thought and kite flying is outlawed.