(I will make a presumption about what your point is...)
I don't actually particularly about the size of the benefits bill. I do care if it creates perverse incentives and some people abuse the system. YABU to suggest that the degree I care about that should reduce because other people are getting money for other reasons (e.g. pensions.)
If the pensions bill were 0% or double the size it is it would and should make no difference to my desire for action to on perverse incentives.
If the abusers are 1% or 99% of the people in relevant subcategories is irrelevant to me. If it's possible to create reasonable reforms that stop them doing what they're doing, I want them added.
For example, someone posted earlier this year about a friend who was planning to have a baby every five years in order to stay on benefits. I have no idea how many people do that and I don't care what the number is. But I would support a measure that requires women to go back to work at most six months after birth, like working people do. And I would support it even if to enable it it was necessary to proved 55 hours a week of free childcare, and so this cost the state a lot more. It's possible that removing the incentive to have children will actually save money in the long-term though. Even if it doesn't, I suspect there will be benefits to mother and child from working/nursery.
(I suspect that someone will be along shortly to say no women has ever done this, in the whole history of the welfare state. Actually, even if that's true, I don't care. I want the rules changed so nobody could do it!)