That's perfectly ok, what you don't have you can't use.
> and that old and slow phones are going to get updates that makes them run even slower.
It would be up to the manufacturers to ensure that this is not the case. One way to do it would be to reduce bloat.
> To top it of, it is not like manufacturers are into throwing away money, so this will likely mean that prices on the cheapest phones will increase more.
This makes no sense: manufacturers are competing with each other, if the prices on the cheapest phones will increase more then surely someone will exploit that.