How do you talk to your parents about getting extra help down the road when they need it which might end up out of their pocket for daycare?

You can only control what YOU do.

If you are unwilling to do what your parents need to remain in their home, tell them straight out " I can't do this any more". They will need to make other arrangements.

I've never found hinting at things to be effective. Saying "no" straight out gets the issue out in the open.

I frankly don't know how anyone can take care of someone with dementia at home unless they have hired help round the clock and even that comes with it's own challenges. There comes a time in the progression of dementia when it is just too much on the family, especially if they are the caretakers. I know some have done it and they must be strong mentally and physically and I am in awe of them. I have already told my husband and daughter that they are to place me somewhere if it happens to me. It is the reality of the disease. This is a terrible disease and will take you down in all kinds of ways from the stress of the situation. I don't know how you convince them if they are set in their ways of wanting to remain at home unless you can get through to them that it will be in their best interest to be in a facility at some point. It's also cheaper to be in a facility than at home, if they are worried about the cost. I hope you can reason with them, but if dementia has already set in, reasoning may have already left the building.

At some point in the near future you will be totally outnumbered by 2 people. What do you want for your life? Do you want to be a caregiver? Have you spent some time reading the thousands of posts made by loving, well-meaning family members who slide into caregiving but don't really know what they're signing up for and completely burn out? Like JoAnn commented, it's not about what your parents want, it's about what they need. And they need more than what you can give them. I also agree to start out with some agency in-home care which helps them and you. See how that goes. Good luck!

What are you willing "not" to do for them.

People suffering from Demenia should not be alone. Do you have DPOA?
Maybe a sit down. Explain that you need to work. That you can't be there all the time so they need to hire someone to do for them what you can't.

There will come a time that your parents will need more care than you can give. It will not be what parents want but what they need. You may want to start dropping hints about an AL. Tell them what you can and can't do.

If they can, or at least dad can comprehend a sit down conversation now is the time to have one.
Do they have the finances to be able to hire a caregiver that can come in? Maybe start with a few hours a week to help out then increase the time to a full day then 2 full days and so on.
Is your dad a Veteran? If so there are programs the VA has that might help out.
Have you thought about Adult Day Care for both of them? This would give them something to do, be with people.
The other thing you need to consider...at some point they will not be able to stay alone in their home. What is the next step? Are you going to move them in with you? Is your home ready for that? Are you going to look for Memory Care for them? It might be wise to plan a year or more in advance.