No, I appreciate that some won't want to outsource, and that's fine if they don't want to and can afford not to. My point was merely that people don't usually need a sahp to facilitate their career progression. Some may want this arrangement though, and that's absolutely fine. We all do what works for our families.
As for the 8-6pm in full time nursery thing, I agree actually that that isn't ideal, but I know very few dual income households who have actually gone down that route. So many people are able to work flexibly these days in one way or another. We never had dd in paid childcare (nanny) for more than around four hours a day when she was very young, and didn't use it at all when she was at school as dh and I were able to work flexibly around each other, but I know so many other ways in which people have organised things.
Personally, I believe that dd gained more from the time that she had with her lovely nanny than she would have gained by having had a few more hours with us, as the nanny brought different skills and strengths which complemented our own. Of course, we were lucky enough to be able to afford that, and not everyone can, but most people find a way of managing things in a manner that works for their family.
My dd is nearly an adult now, but she has had many of the same friends since she was tiny. Some had SAHPs when they were tiny, others had mums who worked part time, others had both parents working full time. Quite honestly, you wouldn't be able to tell the difference between them at this age - none of the different ways of managing the early years appear to have had any impact on their wellbeing and mental health, their confidence levels, their success at school etc. I think we all overthink this stuff when they are little, when ultimately all that matters is that they know they're loved and that their needs are met.