I had the misfortune of doing some work in a mixed shop where most of the Unix servers were running Solaris 8/9 recently. A couple things that led to its downfall:
1. No package manager. And now that there is a package manager, maintainers are few and far between. I don't think I would trust it in a production environment. It's never going to be as robust as dpkg or yum. if they had fully open sourced Solaris, and supported the community as primary, there would be a well-maintained package manager. I doubt Oracle will do anything to fix this.
2. I'm still not entirely clear on where their utilities like grep, awk, sed, etc. come from. All I know is I was terrified to push /opt/bin ahead of /bin in my PATH for fear of breaking something, and meantime I hated the native shell tools. They should have adopted GNU's tools as first-rate. Solaris' tools suck.
Open source won in this area because of its "by developers, for developers" approach moreso than the source code ecosystem.
Now yes, proprietary software like Solaris could, in theory do that. But Sun would have had to double its software staff to even come close to the stability you get out of Debian or Red Hat's repositories.