To address your points specifically;
(1). You only need to connect once to regester it. After that you can run it in offline mode or simply turn off your internet.
(2). Why would you want to resell Skyrim? Fair enough for other games, but I still have Morrowind and Oblivion, and regularly play them, because of mods. I have no need to sell my disc. I bought Oblivion on a Steam sale recently, because I lost my disc.
(3). Mods work exactly the same. The Steam team even went out of their way to make OBSE work with Steam. What more can you ask for??
(4). I wouldn't worry about Steam collapsing anytime soon. But if they do, then they have stated they will remove all Steam dependency. Can you really see them going out of business/being bought out any time soon?
(1). Yes, I understand how Steam's offline mode works. I use it with Dawn of War 2 (the only Steam game that I own). My point is that Steam DOES require internet connectivity, which seems unnecessarily for a purely single-player game like Skyrim.
(2). I just like having the option of reselling my games, so that I can apply the proceeds to my next game purchase. Granted, TES games are generally keepers, but most games are not, and I like being able to sell them on eBay. Steam does not allow that, or for you to even give away a game to a friend or relative.
(3). I didn't buy the Steam version of Oblivion, so I don't know how it worked with OBSE. However, does the OBSE work as smoothly with Steam as it does with a disk version without Steam?
(4). I agree that Valve/Steam is probably in good financial shape for now. However, we just don't know what is likely to happen over the next 5-10 years. Companies get bought out, merged, plagued with corruption, etc. all the time. Hopefully, this will not happen to Steam, but who really knows. Anyhow, I wouldn't gamble on any rumored statement that Valve would "remove all Steam dependency" in the event that something happened to Steam. This is not stated in the Steam EULA, and I imagine that some developers would really have a problem with Valve just removing the Steam DRM altogether and making their games DRM free. In fact, their agreements probably don't allow for this, and instead transfer the game code back to the developer as opposed to making it available without the Steam DRM.
To be clear, the closest that the Steam EULA gets to telling us what will happen in the event of its demise is the following:
2.A. You understand that neither this Agreement nor the terms associated with a particular Subscription entitles you to future updates, new versions or other enhancements of the Software associated with a particular Subscription although Valve may choose to provide such updates, etc. in its sole discretion.
5. You acknowledge that Valve is not required to provide you notice before terminating your Subscriptions(s) and/or Account, but it may choose to do so.
9.C. VALVE DOES NOT GUARANTEE CONTINUOUS, ERROR-FREE, VIRUS-FREE OR SECURE OPERATION AND ACCESS TO STEAM, THE SOFTWARE, YOUR ACCOUNT AND/OR YOUR SUBSCRIPTIONS(S).If anything, the language of Steam's EULA is designed to make sure that you don't have any way of gaining access to your games if for some reasons Valve goes out of business or just decides to change its policies. In any event, it says nothing about unlocking or removing Steam dependency if something happens to the company. That's the single thing that bothers me most about Steam.