So your parents frittered away all of their money on living in the now - buying things without thinking about the future, indulging in fancy things even when they had to go into debt to do so, lived their lives so that the threat of money problems were so terrible that you grew up hearing them arguing so often about money that it is a vivid memory.
Now you propose bailing them out of their too expensive house, so they can quit working "in a few years," meaning while in their 50s, which is super early retirement age, usually meant for those hard working people that save, budget and stuff (i.e. not like your parents). You literally are going to reward a lifetime of their terrible, irresponsible behavior.
And how long do you think they'll last paying for their lifestyle after they're no longer working, even without a mortgage payment? And what about insurance, taxes, cars, eating out, buying things for others (which they shouldn't be doing if they have no money saved for the regular bills anyway), and all the other stupid spending decisions they've made... they're suddenly going to become perfect budget masters that count every penny of their pension/social security, which they won't even be able to draw on until 62-ish and that's if they make the stupid decision to draw early? What are they going to use to pay for their expenses if they're not working and they don't choose to invest the buyout, or draw it down so fast they run through it in a few years? Because you buying their house isn't going to fix things; you're just kicking the can down the road a bit and the problem is still quite THE Problem, and won't get better if they don't figure out how to be responsible, frugal adults on their own.
I get you love your parents. But you are not responsible for their mistakes, nor should you bail them out of easily correctable behavior. Bailing them out without helping them to fix the underlying issues that got them into this mess is like slapping a bandaid on a broken leg - they'll be back before you know it, hand out and expecting more and more from you and soon feel entitled to your economic support. It's called "enabling" and it never, ever works.
They are capable of working? Then they work. They want to spend money, they need to earn it. If they don't have enough? Then they do without some of their fripperies.
Next time they hint that they don't have a dime saved, you ask them if they would like your help in figuring out exactly how bad the picture is, what they could do now - sell fancy house and get a small house or rent a reasonable apartment, work options, how to save and invest, what they can expect from social security, basic budgeting.
If they want help, THAT is what you give them - actual, real help. NOT A BAILOUT. They learn nothing from being given gobs of money or you buying their stupid house and don't think it's going to end there. If they truly want help, then give them real, actionable advice and support. That is what you offer ANY relative or friend. Your wallet is closed, unless it is an emergency. You tell them that you love them, but money creates a terrible mess in relationships and you don't want this to drive a wedge between you and this most certainly will cause long term worry and resentment if you have to provide constant financial support.
And you tell them that you're unable to provide living expenses or or long term financial support. Offer then a few thousand if they get into a desperate situation, but honestly this is not something you need to get into, and you need to make sure they understand that they can't expect you to become their sole supporter due to their lack of planning.
The ONLY time I'd say it was a good idea to give money (large sums anyway) would be a true emergency. An accident, an unexpected and desperate situation that no one could avoid... not this sort of thing at all.