This. ^ It's also highly unlikely the OP's parents would have made 100s of 1000s of pounds on a buy to let house. I have literally NEVER known anyone do this. Many people make a loss actually. Some break even. Some make one or two 1000 pounds a year. 100s of 1000s of pounds on one house? Nope. Where did you get that figure from @Tead ???
They would have been paying many 1000s of pounds over the years for repairs and maintenance, and as the poster I quoted said, they would probably have encountered at LEAST one tenant, if not more, who didn't pay their rent, left with possibly three to five thousand pounds owing, (depending on how much the rent is obviously,) and would have caused your parents a lot of stress. (Then there would have been legal fees/court fees etc...)
Hell would freeze over before I would EVER buy a house to rent out. Heard far too many horror stories.
But yes, as a load of posters have said, stop beating yourself up. There's really no need. Apart from the fact they would NOT have made 100s of 1000s of pounds, they do actually have a mind of their own.
Me and DH changed our mortgage from endowment to a regular (repayment) mortgage, (circa late 1990s.) At LEAST half a dozen people informed us we were 'wrong' to do this, and we were even called 'a bit thick' by one of DH's colleagues. DH wanted to leave it as an endowment mortgage as he was easily led at the time, and felt peer pressure. Not me. I stayed firmly fixed, and said 'they can all fuck off. We're changing it.'
Guess who had the last laugh, when the vast majority of endowment mortgages crashed and burnt some 4 to 5 years later? And hilariously, the naysayers who had derided us and called us thick etc, all sheepishly changed their endowment mortgages to repayments ones!
Different issue of course, but same principle. We - or rather I didn't listen and went and did what I wanted anyway. Your parents could have done the same.