Singarounds - organised by and for singers as opposed to paid-for entertainment - are one place where people can find their feet (or their voice). The informal free sessions in pubs or on the fringes at festivals are also excellent nurseries for talent.
And at home or at work or in the car. I sing nearly every day, but haven't sung 'in public' for many a long month. Maybe I'm lucky in that I work out of doors and alone (apart from a long-suffering dog), but I find that helps.
And, regardless of where anyone sings, there should always be a hyper-critical audience, which is the singer him/herself. That's the most difficult one to ensure, and one which seems to elude many dire singers. Many, I fear, are simply unaware that they are not doing any favours either to the material or themselves. Just as we are all above average in everything we do, so many singers will consider themselves competent to perform in public when, to the outsider, they're clearly not.
But it's difficult to separate the issue of declining folk clubs and poor performing. In my view they are firmly linked. As such I fear the same arguments will be trotted out on all the threads you mention, and nothing will happen. Internet forums do not change people's minds.