You can mod consoles, install unauthorized software on them, even entire new OSes (the original Xbox this was actually very easy, didn't even require hardware mods). PS3s were open for awhile and some organizations even used them in compute clusters.
The fact that SteamBox is open makes it more desirable for some, but it is not a dealbreaker for anyone that I'm aware of. Nobody goes to Walmart and says, I want a inflexible console that I can't install anything on! I don't think PS3's sales instantly exploded when they closed their platform. If you want that so badly, you can just pretend the software can't be changed on it and be none the wiser.
At best, to the end user, if something is open and closed it is total indifference. They don't care either way. The very few that care (like me) would prefer an open platform. I would love to be able to install Linux (or hell, even Windows) on my Xbox 360 for instance.