If I understand correctly, the relationship between Solid and Inrupt Enterprise Solid Server is similar to GitLab CE and the proprietary GitLab server. That is, the main driving force behind both are the same people but their goals are conflicting. The community attracted to the Free Software version have motivations that can (and certainly are) different from the motivations of the Inrupt company which (that’s the definition of a company) is about making money.
That kind of conflict of interest is something I’m watching closely when and if I choose to use a Free Software. In the case of GitLab, for instance, I decided to take the risk that the commercial interests of the company driving GitLab will eventually make it difficult, if not impossible, for me to use the Free Software version. My discomfort grew over the years (a number of essential features are implemented but not published as Free Sofware, such as mirroring a repository) and I’m happy about the prospect of moving to Gitea which does not seem to have such a strong conflict of interest.
In the case of Solid, my choice was even simpler since, to my knowledge, there is no ready-to-use Free Software implementation anyway. But since Solid was mentioned a few times in the context of this forum, I’m sure people have a different opinion on the matter and I would love to hear about it