I think you underestimate how horrifying the prospect of divorce can be to a man; the expense, missing out on precious childhood moments, the embarrassment, the isolation, amongst all the other fallouts. Which is worse? That, or the prospect of a sexless marriage? It's so difficult. At least he is getting some sex. In this situation, of course most men will cling to any glimmers of hope at all. At least it used to be good, right?
Going from an adventurous, satisfying sex life, to missionary only every six weeks. That is not "what she enjoys sexually". That is a woman who has put severe boundaries in place because she would clearly prefer not to have a sex life at all with her husband. She doesn't feel no sex at all is realistic for whatever reason, so this is her compromise, and what she is prepared to tolerate.
Like you say, take it or leave it. No wife owes her husband a great sex life.
I think that something spouses do owe each other is honesty. Why has this changed so much? Does she still love him? Does she want to stay married to him? If yes, why doesn't she care about how miserable he is? Is there anything specific either of them could do to improve it?
I don't think it's really fair to just leave him guessing about all of this.