N0m: There is no abundance (i.e. more than other demographics, namely learning boys) of non-learning boys who remain long-term single or even single until an older age. In other words, there is absolutely no evidence (if you submit otherwise, please do submit it) of non-learning boys marrying later than learning boys or of them remaining single in greater proportions than learning boys.
This is even the case if you discount “fake learners in yeshiva” and disregard them. Regarding those openly not learning in yeshiva are not getting married later or never moreso than learners.
Regarding going OTD (“leaving the fold”), there’s probably some truth to that; but that’s more due to the fact that learning Torah is more likely to keep one in the fold and sincerely observant than one who isn’t learning.