I was around for PB and I do think it brought members in, but most of them didn't stay and post in other topics I don't think. The MN published numbers of members and the number of people who actively post is vastly different.
Everyone always says 'before X, Y or Z it was different and it never really was that amazing, it was just that it was smaller and more people knew each other and some posters became 'Royalty' who were quite often nasty bullies but they got a pass because of their usernames and the fact they'd been on MN for a long time
And your last paragraph. That was going on in 2010, probably before that, and has been a reoccurring thread every week or few weeks since forever basically.
"This site is being infiltrated by men who hate women/are MRAs"
"Why are people who aren't Mums posting on Mumsnet?, it says Mums in the title"
"Mumsnet is going to the dogs, everyone is horrible and it didn't used to be like this"
Dozens of threads like that every year and people always saying 'AIBU gives robust answers but the whole site isn't like that' and people saying 'yes, it used to be great and full of intelligent and witty posters'
It was never that awesome, it's just that a lot of people really loved it at first because that's what happens when you join something, it's cognitive bias. But after the honeymoon wore off and they paid to attention to the shitty bits, they think the site and not their perception had changed.
And on and on and on.