I can actually think of an exception to "...once zealots have established a limit, it is never increased..." The zealots and limits I'm thinking of are population control zealots and their proposed target for the UK population size.
Back in the 1970s and 1980s the UK Greens, or at least a substantial proportion of them, proposed that the UK population should be cut by 20 million, which would be equivalent to the UK population target being 35 million (assuming the population was about 55 million at the time). This Wikipedia article mentions the 20 million cut figure:
I've got a feeling the 20 million figure was official Green/Ecology party policy at one time, but the Wikipedia article gives the impression that the issue only came up in 1989 and was never accepted as party policy. However I think that the figure counts as a limit established by zealots.
In the present day the main pressure group for UK population control is the "Population Matters" organisation (formerly known as the "Optimum Population Trust") whose activities are endorsed by 'distinguished patrons' including David Attenborough, Paul Ehrlich, James Lovelock, Crispin Tickell, Jonathon Porritt and Chris Packham. Their website is: http://www.populationmatters.org/
Their proposed UK population target isn't easy to find on the website. You have to follow the links: About > Campaigns > Call for an end to population growth
The last sentence in the 'Call for an end to poulation growth' article is "For the UK, we ask the government to commit to keeping the population below 70 million."
This 70 million target population figure (which is higher than the current population of about 62 million) represents a doubling of the population target the Greenies wanted over twenty years ago. The Labour government (Phil Woolas) actually accepted the 70 million cap back in 2008, but the Coalition government (Theresa May) rejected the cap last year.
The current position of the Green party is that they have no population target, as the Lefties that now dominate the party do not want to offend immigrants (the people most likely to be affected by a population control policy) in any way. Also there isn't a UK-wide Green party any more - it got split up to take advantage of the tendency for the Green vote in Scotland to be nationalist.