Good- we need professional landlords. We need the amateurs to leave the market and allow families to buy their properties
I'm a professional LL, as it happens - but that's irrelevant. If LL sell up, there's no guarantee at all that they will sell to other professional landlords. They may well sell to unscrupulous amateurs who tick the various boxes but find ways round the law. Section 8 will become the new Section 21. So if the OP were a LL, she could quite reasonably argue under Section 8 that any tenant introducing a pet into the house could be evicted on the grounds that it would make her severely ill if she needed to visit the property for any reason. This is obviously a hyperbolised example, but this sort of thing will be exploited all the same.
LL could sell to investors who want to use the property as a Furnished Holiday Let - thereby taking it out of circulation for 'normal' people. They could choose to do this themself. They could start to let to students instead of families, in the pretty certain knowledge that students will stay for a maximum of two years - and if they get jobs and can carry on paying the rent after that, a LL will be delighted.
The more LL sell up, the more the supply of rental accommodation will reduce. So the prices will go up, and it will become more difficult to find/afford what's available. The supply of accommodation available to be rented by individuals/families (as opposed to holiday lets) went down 50% between 2019 and 2022.
It's also worth bearing in mind that 88% of tenancies end because the tenants want to move on, not because the LL kicks them out. Most evictions are the result of antisocial behaviour on the part of tenants (or, equally, because the LL wants to sell up). People like to act as if LL were the Devil and all tenants are whiter than white; neither of these things is true. There are good and bad LL and good and bad tenants. So, as is so often the case, we're talking about a marginal number of people. The overwhelming majority of tenants will remain completely unaffected by changes to the law, other than having to pay higher rents as there being a reduced supply of rental property availabler.
As for the idea that families would be able to buy the properties that LL sell: if they could afford to buy property, and were able to obtain a mortgage, don't you think they would have done that already? Nothing is stopping them. There are plenty of houses available already.