I agree with the OP. I personally do not think any arguments made against this are valid. I'm not sure if it's mob mentality but I don't see how it's a bad idea. I don't think it will happen (at least not with ESO) but nonetheless I think an offline RPG as expensive as ESO with story and collectibles would be awesome.
As far as arguments against the OP's suggestion go; I don't see how the 'What about group dungeons?' 'What about PvP?' are helpful. It would be common sense to assume the OP isn't asking for online reliant features to be available offline. When it comes to Skyrim or other TES games being suggested as an offline replacement I think they are apples and oranges. ESO is not Skyrim. There are similarities but I don't think they're comparable as a whole within reason. That and the fact that I am quite certain anyone wishing for and offline ESO game has already either played other TES games until they are dead or are not interested in them due to the different gameplay mechanics.
ESO has many amazing features and a large world to explore- many of the adventures you can do solo without any issues. From loot drops to dungeons- it isn't required that you interact with other players or purchase from the guild traders to level up for progress (even though interaction with other players or making use of online functionalities may be advantageous) I think it's easy to see why someone would want to be part of this and how they may feel held back from the experience of ESO many of us with secure internet (including myself) can take for granted.
A massive open world like this with all the side quests in the gameplay style of ESO would be amazing. That and the graphics are beautiful as well. There is plenty of content and so many different experiences to be had.
TL;DR
I don't believe ESO will ever be offline but I do think an offline game that plays the same way, with the same quantity and quality of content that ESO delivers, would be fantastic.