I suspect that properties will either be bought by a homeowner, thus reducing rental stock (as you say), or will be snapped up by big corp. where rents could (and probably will) be increased substantially (as you also say). They will also have a very powerful voice for lobbying the Government, unlike small private landlords.
I've come across so many posts feeding into my Facebook about families being given eviction notices from their landlords and they can't find anywhere else at all, never mind something affordable. It must be truly awful for so many people.
Some points from the new Act, if rent can only be increased once a year, then will interest rates only change once per year? If a tenant is allowed 3+ months of non-payment before legal proceedings can commence, is there any legislation allowing the landlord to not pay the mortgage for the same length of time? Many people I know who are "landlords" only have one extra property, usually inherited from the death of a family member. Almost all of them are having problems with their tenants not paying and it takes forever to go through the courts.