Yeah, there is a future that is not represented, so there is no reason to assume any downward trend. Plus he only sampled things for which there were open source alternatives (which are also questionable: e.g., AutoCAD - a full featured CAD tool versus Open Cascade, a C++ lib with no UI? Really?!).
The proper way to do such an analysis if different on almost every choice the author made:
1. Randomly, uniformly sample products/service over all time (somewhat difficult). Or better yet, make a cutoff date far enough the past to hide the fake triangle due to items being release last year.
2. Find OS alternative, it if exists, and most importantly, decide on some level of how much is is actually an alternative. Almost nothing on this list is an actual alternative, any more than a text editor is a MS Word replacement.
3. Analyze more carefully.
Post selecting products that fit a desired outcome, ignoring lots and lots of other products, ignoring the time bias, and calling things replacements that are not, then drawing these conclusions, is really bad analysis.
>it still takes years until it can outright displace the proprietary champion
How many items on even this biased selection list displaced the proprietary champion by killing their market? None? A few proprietary products died well on their own. And in this time frame, how many open source projects died?