I feel they have found a strong cast with great singers, which is absolutely essential for this material, so this would be the opportunity. Because if they don't do it, and the show stops after a year, there will be nothing left once again. Only a possible future film version and we all know what kind of people they will hire for that. (the terrible singing which made the les Mis film end up in the $1,- bin at wall mart after a few weeks, while the 10th anniversary dvd is still popular after 15 years) At least we will have a document that does the material justice.
Such a good point about the longevity of the concert/anniversary production DVDs. I bought the Les Mis film on dvd months ago after seeing it twice at the cinema, I doubt I'll watch it any time soon, mainly because of the inferior singing quality to the concert productions. I have the 10th and 25th anniversary recordings which are my favourites and easily the best after the original soundtrack.
I hope they do what you suggest and film it at the end of its run.