I expect that on average, 2016 attendance will be higher, and if there is a drop off, it will first show up in 2017. My reasoning is that we started the season with 7-10k season tickets sold, and kept increasing it during the year, ending with the aforementioned 20k. Even with some non-renewals, we should start next year with at least 17-18k (maybe even more) which is a much higher base than the start this year. Since all sold tickets are counted, our single game attendance cannot ever be below the STH figure. Note that the lowest figures this year were Games 3 (20k), 4 (21k) and 6 (23k). We never got that low again, not just because the single game sales increased but (I think) mostly because the STH base kept going up.
So if there is a downward trend, it would have to be in 2017 if a second disappointing season, coupled with no news on the new stadium, causes a higher rate of non-renewals for season tickets. OTOH, if next year the club does better and/or a new stadium is announced, I think the attendance figures will at worst plateau.