What about git? Wasn’t git developed exactly as an alternative to a commercial product? Git is probably the most successful open source alternative software, so much that it completely obliterated the competition.
It hasn't completely obliterated all the competition, but most of it, and those that aren't dead are on life support: in the open-source domain, CVS is mostly dead, SVN isn't far behind, and Mercurial is barely hanging on. In the proprietary domain, it seems only stodgy long-time enterprise customers still use Perforce and ClearCase, and MS's SourceSafe seems to be dead (and MS even bought GitHub).
Linus was pretty steamed at the participants in that debacle (especially Tridge for some reason) and named his replacement for what he thought of them. Now you know.
The reason was that Tridge was trying to build an OSS alternative to BK.