Most flipper/speculators do not have 10 or even close to 10 mortgages. Most of the people with 10 mortgages were/are investors, who have a vested interest in making their properties nice and rentable, providing a good source for habitable and quality housing. There are actually not a lot of people investing in middle-tier housing (most investors are either in SFR/condos or companies with 50+ unit apartments). So in my sphere, the structure is pretty good for supplying housing (and not necessarily encouraging homeownership, but providing housing).
And honestly, if it weren’t for many small landlords, the only affordable housing would be by corporations, who only tend to provide high density apartments because of economies of scale.
In any case, the 10 mortgage limit was probably some kind of arbitrary number and was probably predicated on the idea that people were responsible and would honor their debts. However, with the current environment and the advice being “walk away from your mortgage” being prevalent, it was probably a wise idea to at least limit the number of mortgages. Still, in my experience, the rule has the effect of punishing those who have followed the rules and been honorable (as many of us here can attest to) – namely the small and getting started investor.