The OS i one thing and there are many applications (like Ovi Maps, Mail for Exchange, Internet browser, etc) that are not part of the OS and that are not necessary Open Source. Even if the OS is Opne Source, is there a download link from where you get an installation USB memory card from where you can install simply with no issues the OS? And even if all of these are available, how many people do you think are reinstalling Windows on their computers - it is less than 5%. The point is many people, the majority, will rely on what "comes with the device".
The good thing about this is Nokia will provide beside telephony a large spectrum of "communication" from email to VOIP. On N95 that I have there is a very good and customizable VOIP client, but on N96 and 5800, to please the operators, Nokia stripped down the phone VOIP client. This is what happened with Symbian to please operators. Good news with Maemo and N900 will not be the same path and Nokia wants to deliver a full communication device with no operator-imposed restrictions. Hopefully the next Maemo device after N900 will step in the same shoes...