I highly recommend Stephen Tetlock’s book, super forecasting, who is the sponsor of the project you mention.
One method of forecasting that he identified as effective was using a spreadsheet to record events that might occur over the next 6-18 months along with an initial probability based on good judgement and the factors you quoted. Then, every day look for new information that adjusts the forecast up or down by some, usually small percent. Repeat, and the goal is you will trend towards a reasonable %. I omitted many details but that was the jist.
Now, that’s for forecasting on a short ish timeframe. There is a place for more open ended reasoning and imagination, but you have to be careful not to fall prey to your own biases.
This particular forecast of OPs feels like it is ignoring several long running trends in technology adoption and user behavior without giving events that would address them, and forecasts something they care about doing better in the long-term, a source of bias to watch. I tend to agree with you that I think elements of this forecast are flawed.
So first there is a difference between reduction of meat products and an elimination. Having people consume less meat is good and helpful even if they don’t cut it out completely.
Second, as a vegetarian, I don’t understand what you mean by producing a bunch of monocultures. Do you think vegans just sit around all day eating avocados? I eat very little dairy or egg, and my diet consists largely of beans, rice, chilli, bread, stir fry, tofu, peanuts/legumes, veggies, baked potatoes, sandwiches, etc. I eat a large variety of staple goods cooked into a variety of dishes from around the world, and classic American fare, just without meat. Avocados and other resource intensive crops like almonds are a minority of my diet by a large margin. Things like beyond meat is also an infrequent treat.
Edit: here’s a decent article. https://www.nytimes.com/article/plant-based-diet.html
Referenced research: https://www.nature.com/articles/s43016-023-00795-w