NB, virtually all of the plots available now in areas with established sites are vacated because the previous tenant was evicted for failing to cultivate, it, or the council is desperately making available plots that were left for decades to try and clear waiting lists.

Some places are managing the 12 or more want in the parish with no provision and usually that means a grass or cultivated field with a community pulling together.
My council spent over £10,000 to bring an abandoned site back into use and that involved big tractor mounted; brush cutters, ploughs, rotovators and weedsprayers.
To clear one plot based in that is £200, I reckon it would be much more.
Associations, well ours has been around for decades, we only get about 100 of the near 1000 plotholders in the town joining and about 1 per year becoming actively involved with running the association.
The eviction process takes time, you have to be fair and take circumstances into account, he who has a hip replacement, or breaks his wrist for example.
My opposite neighbour is a bit slash-b-burn with dog daisies all over his plot mixed with couch and spaces cleared for specific crops, it looks a little as I imagine your garden(except the couch), stuff mixed and dotted around. He produces a lot and wins show prizes, buts strictly he could be eased out for untidiness.
Unless you spend an hour or two every week inspecting every plot and hassle people over individual weeds, becoming the weed police, then allowing for nature spots, weed mulches, green manure, holidays, etc then it takes time for it to become apparent that a plot is being neglected.
Then you have to make contact, give a verbal warning, leave it for a reasonable time to change before the council gives notice with another grace period, followed by "you're off".
As for some of the reactions I get when chasing people up over their untouched plots, well the woman who was allergic to sunlight sticks in my memory, took three years for the realisation to hit her that siting a log for her child to sit on for the ten minutes a month they spent at the site.on cloudy days did not constitute "cultivation".
I both try to give open advice (green & chemical) with pros & cons, plus make introductions to old boys & girls on site already, people often don't want it, they have ideas, sometimes from tv and what do I or the plotholders of thirty years standing know that Monty Don didn't tell them......
Sadly the world or approach you describe is idealistic, expensive and, given modern attitudes to "my rights" a tad naive. While rarely it is as you write, I wish it were all the time, as it would make my site agent job a lot easier, as well as newbies having a better start.
I've let three clean starts and only one of those was someone cutting back before age related infirmities really affected them, the others were sadly, a sudden death and a move hundreds of miles away.
I'm in a rugby club as well with over 1500 members, nine of us turned up for a working party to do repairs an etc, pretty much the same group as turned up at the last four working parties and my wife assures me I'm not afflicted by BO or such a miserable old g** that people avoid me.