This isn't a black and white situation like so many are making out.
Some details are lacking in the post, but I'm basing my comment on the assumption that the OP is male, and the SAHP is his wife.
So, in terms of retirement funds, the OP should have been putting away money for their joint future as the wife has given up her earning potential to raise their joint children.
Now that those children are grown up, she's still a parent, but with much less parenting duties, she's likely in menopause, her body tired and exhausted, the thought of working full-time again will likely overwhelm her and fill her with dread. There is also the aspect that at her age and having been out of work for I'm guessing 20yrs or so, it won't be easy to get work and she may end up with minimum wage work.
It depends on her health but if she's fit and well, I do think she should be doing all of the housework and cooking etc. if the children are now adults and you're working full-time. If she's not in great health, then sharing that seems fair.
I'm a SAHM BUT I did invest prior to leaving my career so I do have an income from that. Our children are still young children, but I have started my own business for some income as well as the income I get from my investments. We do share housework, the way our household is, I'm out of the house a lot more than my WFH husband due to our children's busy daily activities. I won't be going back to work full-time when they are adults, I'll be approaching 60yrs old by then and my body is already exhausted with peri-menopause. It will be too late to go back to my career by then and my husband would prefer I didn't work NMW jobs. I'm not saying this is how I feel, but he sees it as 'degrading' as I was a higher earner before children - again, this isn't how I see it, but he'd rather I didn't work unless it was on great money, which is unlikely now.
Anyway, a sit down chat with your wife to see if she can pick up a lot more of the housework and cooking etc. to ease the burden on you would be a good place to start.
What exactly are your expectations? It would be difficult for her if she's in her 50's/60's to get back into full-time work both physically and trying to find suitable employment.
ETA I am aware that the sexes might be the other way around, and that may change things slightly then if you're the one going through menopause etc. so could understand why you'd want to share the financial burden then by you working less and your husband picking up part-time work to help with that. It's quite hard really without knowing a lot more details.