It is far too complicated to have a simple answer like it's always/never right. Social care needs FAR bigger budgets, more staff, infinitely more resources. That is the root of the vast majority of the problems.
What are the alternatives? Sometimes there are none. People cannot keep having more and more chances when they are not keeping their children safe. Indeed, when they are the cause of the danger.
It might well be that an abusive parent is acting out things they themselves have suffered as a child, but that doesn't make it okay! What, leave the children there so they can go on to repeat the cycle too, that works? And when somebody does not know normal, basic things about keeping a child safe we can't just say oh but they didn't know, never mind. You either have to give them adequate support (and adequate is HUGE. We are not talking about somebody being unaware of a cultural norm about leaving children alone when they are reasonably safe, we are talking serious neglect and a total lack of awareness of safety or hygiene.) or remove the children. Perhaps not removing contact entirely, although that has downsides in itself of not allowing the children to fully move on.
Warning upsetting content in this paragraph. Skip if you want.
In the USA, parents convicted of abuse or neglect are sometimes offered parenting classes rather than prison. In the book 123 Magic (which is just an ordinary parenting book with advice on discipline) the author describes two such parents: One had poured bleach down her child's throat and the other had set fire to their child's comfort object in the kitchen sink with lighter fluid. As a punishment. These are not parents who "don't know better" or have got to the end of their rope - that is serious intent to harm and shows a frightening lack of empathy, possibly at sociopathic levels. Some parents are addicts and will always put their addiction over their children. A friend of mine moved into a maisonette which had previously been some kind of crack den or brothel, and there was a small cupboard about the size of an airing cupboard. The house was empty and had been cleaned, but inside this cupboard, they found hundreds of shoe prints and scuff marks around halfway up the walls, the size of a small child's feet. Around and around and around.
I am sure that anybody who has had any contact with children's services could tell you worse. It's really, really, fricking dangerous to assume that all parents are basically good people and can be good parents if they are given the right support and enough information. That is a fallacy - an understandable one, given that we tend to assume that others think like we do. We all make mistakes, as parents, and learn and improve on them - it's easy to assume that that means some people have more learning to do than others, but the mistake is assuming that everybody wants to or can learn and improve. It's easy to think "But if I was in that situation and I genuinely didn't know, it's not fair" but the reality is different. Social services cannot hang around a family forever and keep on holding their hand, their budgets are too stretched. They have to be sure that the parents are able to keep their children safe, care adequately for the children and not harm them, without needing to be told how all the time before they allow children to remain in a family.
Remember some parents are abusive partners. Some parents have personality disorders. Go and read the Relationships board for an idea of what that is like to live with. And for a child dealing with that kind of treatment, it is worse. An adult has some sense of agency and some hope of leaving. A child will assume that their parent is absolutely right, and accept the treatment as their due. They have no power to leave unless another adult intervenes. If their parent is spectacularly unbothered about protecting them, the other adults around might not have an interest in intervening, instead seeing it as carte blanche to step in and abuse the children further.
What I do think should happen is that parents should be allocated an independent advisor who they feel is on "their side" (of course it's not about sides, but families often feel it is) who can help them work with children's services and understand where they are coming from as far as is possible. I do think that a lot of parents are baffled and terrified by the involvement of children's services and tend to react either defensively and aggressively - because that is the only tool they have to deal with what they perceive as a threat - or panicking, running away, hiding, ignoring. If you have not experienced children's services involvement it is very very difficult to imagine, but let me tell you, suddenly realising that somebody has the power to take your children away, having them interested in you feels like watching your child run into a road. We cannot and should not expect parents to react in a calm and rational way, because it activates all of those fight or flight, mother bear, protect my babies instincts, (even if they were not there when their new partner or their dealer was abusing their child, the threat of losing a child is often more base and primal) and this instinctive response is often totally opposite to what a social worker wants to see.
Of course this is expensive and won't happen any time soon. But I think it would reduce the need for adoption and help to ensure that when it happens it really is the best outcome for the children. And overall, I agree with the OP. It is the best thing in a lot of cases. Perhaps it could be handled better, but I really think children's services are doing the absolute best they can with the caseload, staffing and budgets that they have: Too much, not enough, not enough.