All modelling is limited and error prone, I agree. That is why I started off by saying "the model estimates": it is an estimation, not a certainty, by any means. 'The most optimistic considered' is standard scientific/mathematical jargon. They calculated the range for things like R values using the best available data and the best available analysis. If they used the lowest value for R they get the most optimistic considered, and the high end of the range you get the most pessimistic considered.   Of course, it is possible the R is lower that the low end of the range: it is possible it magically becomes 0. Nevertheless, on all available knowledge and testing and analysis it will not be lower than that used. Equally, it is possible R is higher, but they only considered the high end of the range of the analysis.

People are free to talk about anything they like and to disregard the model if they wish.   Johnson certainly seems be behaving as if he is. The hot topic of the day may well be masks, and opening nightclubs and all the rest.   But the warnings of the costs are published on the governments web site. Not talking about them does not mean the warnings are not there.