Lots of people have thought for a long time that the housing market surely has to fall as housing costing 9 x average wage is not sustainable but it hasn't. People (e g. banks) are predicting falls of up to 30% now but who knows if it will come to pass. Someone might buy and experience negative equity or someone might wait and watch prices continue to rise. You never know which until after the event.
We bought in Sept 2007 just before prices did crash and it was over a decade for our house to get back to being worth it's purchase price (Cornwall).
I agree that you don't know if your parents would have had a run of terrible tenants trashing the place and not paying rent and sold up years ago. You don't know that it would have worked out for them.
I don't think you could gave in good conscience simply recommended your parents to buy. Prices might have fallen. Tenants might have been terrible. Legislation might have been punishing (it has been for some higher rate tax payers with mortgages).
I think the best you can do in similar circumstances (big decisions) is draw up a list of pros and cons of the potential risks and benefits and let the person decide what to do based on their appetite for risk.
Re investing in the stock market - my sons child trust fund has been invested and added to since he was born in 2009 and is barely worth any more than the cash value invested. There have been times in the last couple of years it was worth less than the cash invested. There have been good and bad years and not helped by a fee that is higher than some at 1.5%. You live and learn but the fact still doesn't tell me whether stocks will outperform cash in the next 10 years or not. No one has a crystal ball.
Be kind to yourself. You tried to protect your parents from potential risks when the benefits were unknown.
Someone else could read your thread and encourage their parents to invest in buy to let that loses money over the next decade. Hindsight is 20:20.