Based off the Blockland Forum's statistics, we're averaging a -35.9% decline in activity with every passing year since 2016.
Using this you can make a somewhat accurate prediction of the following years.
2020 had 1,777 new topics
2021 will have roughly 1,139
2022 ~730
2023 ~468
But of course, predictions become increasingly inaccurate the further into the future it is.