My memory is probably a little earlier, but somewhere around the 2014-2016 range, when growth stops on that graph. Work stopped using the paid tier at the same time as they were looking at a huge 200 or 400%+ price increase. So I imagine it started quite an exodus of non-enterprise level users...

Work ended up finding something better at less than the Basecamp old price, let alone the new ridiculous one, though we did have to try out a fair few to get there. :)