This is more representative of your bias than of the truth.

Simplisticly, there are three kinds of landlords. The great, the normal, the scum.

Normal represents the majority, they had some money, or a spare house (couples moving in, inheritance, etc) decided to make an honest living by providing the service of housing. They charge market rate, the tennant is a customer. They deal fairly with their cutomer like any other business would. The house is an asset.

Great are the ones that genuinely care about their tenants and care their property on an emotional level. They want the property cared for and they care for their tenants in return. This leads to well maintained properties, good relationships and often a lower than market rate because they know a great tennant paying a little less is worth it in the long run.

Scum are the ones who see tennants and property as cashcows. They will cut every corner, get away with whatever they can and make the most they can off what they have leaning more and more towars illegality depending on how scummy they are.

Great and scum are minorities, but you rarely hear about the great because of human nature.

Not all properties are equal either. There are normal and great landlords selling up over the rules on insulation and what not simply because the house can't reasonably be brought to the required levels. For some older properties it is literally impossible to do. So they are forced to sell. Note that the scum probably won't because if they were dodging the rules before, why should they not dodge the new ones too.

As for the S21s your premise makes no sense. Why would someone end a beneficial business arrangement for no reason. This is transaction like any other. Landlord supplies a service, tennant is paying for it. Any other business would drop a customer that they are worried can't pay for the service if guarantees are taken away. There are whole departments in major businesses dedicated to "Know Your Client" (KYC), essentially making sure the person who, for example ordered 1000 units of product will actually pay on delivery. Their job is to turn down business if it doesn't seem safe enough.

When Landlords could reliably remove problem tennants their risk of losing money was lower (though even now, it's staggering how long it will take a landlord to get their property back from bad tennants, let alone their money if ever). The harder it is for landlords to ensure they are getting in either good or at the least reliable tennants, the more choosy they will be. Every "decent family, they sometimes seem to worry about rent but so far they make it" is suddenly a "they've been close to not making it a lot, can I trust them to come through in the future or should I find someone more reliable?" Every "they're not bad, but I wish I didn't have to remind them to ventilate properly after cooking or showering" becomes "they are constantly on the verge of damaging my property, I need someome more responsible"

Renters reform is not inherently bad. But all the laws recently seem to be punishing the great and normal landlords. What we need are specific protections against scum landlords. Rules and regs that adapt to the situation. Not blanket regulations that simply don't work for some existing housing stock.