I don't know about Spitz (now no longer a venue) and Cargo, but the Electroacoustic Club at the Slaughtered Lamb is very, let's say, "selective" in what they pay and to whom.

With all those venues, their admission price varies according to the stature of the performer. (As you'd probably expect it to.) But to properly address the problem highlighted by the mag's article you'd need the "premium performer" prices to be the default. (I'm not saying that this isn't workable: it might well be)

There are clubs that pay well, there are clubs that don't pay at all. There are clubs that will put you up, there are clubs that won't. Some are great publicists, others aren't. Some clubs pay support acts, some don't. Chacun a son gout. I suspect most performers doing the rounds probably get the measure of the lay or the land pretty quickly ,and adopt a "swings and roundabouts" attitude. As you say, performers vote with their feet, but they also quite often are content to play quite crappy venues, simply because they know there are good ones towards the end of the week.

This is not a peculiarity of the folk scene, it's universal.
It is the same across all music genres in the UK: rock, jazz, funk, indie, hip-hop, you name it.

I think all of Tom Bliss's suggestions above are very good ideas. Were they to work, they would generate a national folk network that genuinely could sustain professional careers entirely within that system, utterly independant of the market forces that conventionally sustain professional careers in all other forms of music. That would be unique - and the envy of other niche musical forms.