This post is a good example of a younger person's view of what an older person should want based on their own life view.
Not specific to your MIL @SomethingYum but the things that seem important to happiness at 30 are not always important to someone at 55, 65 or beyond.
Botox, bikinis, heels, work, make up might seem the gold standard at 40 but post menopause, with 20 years more life experience other things are often much more profound, satisfying, sustaining, interesting and rewarding. Not in every case, but often. Time becomes very precious and getting biking ready or worrying about wrinkles can seem a very empty way to spend it.
I'm in my 60s and have a life full to the brim with hobbies, friends, family, love, dogs, sea swimming, hill walking, volunteering, gardening............but heels, botox, bikinis - fuck that.
You might not know why I would do that to myself but if you're lucky to live long enough you might find out. It's a joy.