No need to argue....it is ! There is virtually no way anyone will invest and earn better than mortgage rates without taking some capital risk. As others have said, a quickly diminishing and then paid off mortgage is very liberating.
The Fundsmith equity fund has performed fantastically well - I put some dosh in it in September 2014 for the first time, that's about 75% up on today's valuation.
The argument is, that investing in equities is HIGH risk..That is a very sweeping statement.