“……they buy a house that costs $500k in TODAY’s market.”

Please excuse the question, but why such a nice house? TG just bought a place that was under 300k if I remember right and it sounds pretty awsome. I dont think that guy has bad taste, so I wonder why you need such an expensive house. An intergenerational loan like you describe is alot easier on the parents if it is 250k (+your 50k saved from screwing the bank) than if it is 500k.

The other question you need to ask is how long your dad wants to keep working. If he wants to drop dead on the job, and is making 100k, then it isnt such a big deal. If he wants a long retirment, then that is a different story.

Stop looking at this from your point of view and look at it from theirs. If it is the best thing for them, regardless of your interests, and it helps you too, then great. If not, then find a new scheam.

BTW, my parents have done something similar with my brother (he was buying his first house), and it has worked out for everyone. It can be a good idea.