This is the right place! People come here to ask for marriage advice, rarely to talk about good and happy marriages. So much, that sometimes it is downright depressing. Which is not to say, you are causing that problem. It is more of the overall feel, some days, for me, anyway. (Explaining why I tend to slip away from this forum!)
So, you are saying in old age, your wife, who never wanted or enjoyed intimacy is even worse? If she is past menopause, desire does go down tremendously, as estrogen in particular leaves a woman's body. So, in that sense, your wife is correct. But, if it was always like that, don't be surprised.
Which leaves you wondering about what marriage is about. If it is just about sex, then by all means, walk through your "go, leave, cheat" list. Figure out which one works for you.
I would hope you would not be so shallow as to think all of marriage happens in bed. If you have children, have you thought of the impact on them, if you leave? If you cheat, always think about venereal diseases, and then giving them back to your wife. As for staying, perhaps you could remember all the good things, and good times with your wife. Take her on a cruise. Not to get her into bed, but to be with her and remember that love is about the person, not just an act.
As for you, perhaps you have just not faded at the same rate as your wife, and you always have a higher libido. Again, that is not a reason to end a marriage, nor to cheat.
I hope you will seriously think about ways to rekindle the romance, which may or may not lead to more intimacy. Because, you have committed to this woman, and a natural and normal reduction in her sex drive is simply not a Biblical reason to divorce and leave her alone.