My Father is well into Stage 5 Parkinson's with dementia. What's next?

I suspect what's next won't be pretty, nor is the disease. I knew a man who lived 12 looooooong years with Alzheimer's.

Do you have an eldercare attorney? Worth some money spent for advice and financial assurances that Mom is protected. Sounds like she is concerned about how she will be provided for should Dad need lots of money spent on medical care, so she's taking on the role 24/7 to protect herself. Could also be that she doesn't want him to ever be removed from the home, but does she understand that there are trusts that could be set up, to save 50% of all their assets for her own needs, and he could even use up all his half and be on Medicaid, but no one would take her half of the money or her home or her car as long as she were living in the home? Does she understand that Medicaid even pays for in home caregivers, though only a certain number of hours per day? But if they have assets, and a trust is created, his money is separated from her half and she is protected, so then, it's easier to see that spending his to help with his care, will not hurt her in the end. Seems like you need someone to help have a heart to heart counseling session with her, to help her state what her worries are, and also to help her see that many 24/7 caregivers, who do not accept help, so that THEY can take care of themselves, end up either ruining their own health or dying first....which does nothing to help the spouse who needs the care. Does she have POA over him and his affairs? She needs help, but from people who will listen to her needs, concerns, wants, and help her understand that she CAN have help with him, on her terms, and with her still in control, BUT, able to plan time to care for herself.

It is about her. The caregiver is the most vulnerable person in the caregiving relationship.
Your mom has shown you that she will continue with things the way they are.
You have the advice of experts that the current situation is unhealthy for her.
The next move is yours. Take charge. Either hire help to care for dad (whether mom likes it or not) or look at assisted living for dad (with or without mom's participation).
Also, visit a reputable elder law attorney with your mother to look at their finances. Worrying about what might happen to their assets doesn't do anyone any good. An attorney can tell you exactly what will happen and set them up for a best case scenario.

Your father will die, it's just a matter of time. My husband with PD just did after 5 years of end stage PD with the last 6 months very intense. Your mother will be bereft and alone with no one to care for and unable to focus on what she needs after so little time taking care of herself. The dementia makes your father very unpredictable. And in my case, he became delusional and would get up to walk and fall. Went to hospital and they put him in rehab. Rehab totally unable to care for him as intensely as he needed. So he fell again there and was sent to hospital again then back to rehab. Doctors at hospital recommended hospice so I said send him home for hospice. They were wonderful, so experienced in handling his (and my) needs. The dementia is what finished him because he no longer could recognize what was safe and what was not.
He died here at home with me and his son at his side. He was responsive to us until the last 3 days.
Give your mother the support she needs, don't judge her. It's very hard to watch your dear partner of many years disappear.

dragonbait- good comments. Unfortunately some people refuse to accept that help is absolutely needed. One serious injury to the caregiver spouse may mean "game over" for both people. For justhere- getting some help from paid caregivers would give your mother a little relief but it doesn't help with off hours. Proper assistive products could make a huge difference in keeping your father upright (and reduce risk of injury to your mother). Having an OT do a house assessment and offer suggestions might be a way to get mother's attention. Whatever money spent on paid help and products is peanuts compared to the costs of extended care (or the emotional/financial costs of an injury). It is a generational thing too- what we often see are the "kids" calling as they see their parents ready to crash and they want suggestions to avoid a coming disaster.

My husband has Progressive Supranuclear Palsy which manifests as Parkinson-like symptoms and dementia. He falls a lot; and, when he fell and knocked me down, I moved him into an assisted living facility. After three months—and three trips to the ER—they insisted he be moved into their "memory care" unit where they could supervise him more closely. Yesterday, I completed the paperwork to enroll him with hospice. I'd say you need some allies. If your father's condition is sufficiently advanced, hospice could be a resource. One thing on their menu is the services of a social worker who could help deal with your mother. You may be able to get a referral to a social worker through the hospital they prefer. The services of a financial planner could be helpful to work out how best to use their resources to ensure there is enough to care for both your parents. (Find one you pay by the hour. Most banks have FPs on salary who will work up a plan without charge, but they want to sell their services to manage the money for a percentage.) For whatever it's worth, I know all about the anonymous "people." I hear from them every now and then. I think they live in the woodwork or something. Part of the baggage of the first born is feeling overly responsible for just about everything.