Have a few thoughts.
Rate : Assume your not getting a fixed rate for 50 years, rate would be terribad if you did anyway, so in reality its not really 50 years for the majority who would switch at some point anyways and could take a shorter term based on changed circumstances, assuming they could have course. Which brings me to :
Capital (LTV) : I haven't seen any charts for % mortgage repaid over 50 years, but its still going to add up. I would guess that around 30 years about 50% capital repayment has been made.
50 years of typical house price inflation, say 50% paid off original borrowings at 30 years, by year 30 the LTV would be low, potentially very low.
30 years of even 2% average inflation of wages is 81% increase, if its 3% its 143% increase (very sensitive over this length of time).
My major concern is that I cannot see how children would be able to get their own mortgage if they were tied to the parents still outstanding after say 20 years.
How would banks / building societies assess the risk fairly.