By that time I'll be almost 50 and my sex drive will be gone ...
No. Trust me, it will still be there, just as strong as ever. I speak from experience.
There are two really bad things about a sexless marriage: 1) you feel horribly unwanted, undesirable. 2) you're married to someone who, in addition to not desiring you, doesn't even love you enough to want to make you happy. The second one is worse. She's not even a good friend. She knows she's hurting you and can't be bothered to step up.
If I were you, I'd start talking divorce right now, for reason #2. Only this time, don't let her call your bluff. You'll find that her sexual interest is suddenly rekindled. I think some spouses have to realize what they're losing before they can appreciate what they have. At that point it's up to you if you want to continue to divorce, or whether living with this sort of "blackmail" arrangement is palatable, just for the sake of keeping the family intact. But in order to keep her interest, you'll always need to have the spectre of divorce looming in the background. But your threat should not be an empty one--you actually need to mentally prepare yourself for divorce and get into that mindset.
I will say this: Once your threat of divorce becomes real, you will have control of the relationship. I think this is a good thing in your case. This will change the dynamic and cause her to respect you more. The respect results in more attraction. You'll have to gauge how much of her newfound sexual interest is based on the "blackmail" aspect, and how much is based on the newfound respect and attraction.