I think that once the initial relief of being married to someone who was kind-hearted and, within his means, generous, wore off, Wallis got bored with Ernest, especially once she met the Prince of Wales and was introduced to a different world (and someone who was besotted with her). Ernest seems to have been a very decent man, but fairly quiet and serious-minded and, though he was comfortably off, he wasn't rich. Yes, they might well have stayed married if she hadn't become involved with the Prince, but led fairly separate lives (especially if Aunt Bessie continued taking Wallis off on trips).
Ann