Playerbase
The most important issue to note is the effects this will have on the number of people playing as well as the kinds of people playing - for everyone that would make the argument that you do not need a large amount of people to enjoy the game, this is just wrong. There is a reason that the games that sell well stay ahead, capital, they have the money to invest back into the game and in turn provide more content, more staff and all around a better experience for everyone playing the game; you could be as shallow to simplify this to 'the more people there are playing, the better the game content will be'. The other side of this comes down to the types of players that will be playing this game if there is no competitive long term PvE content which will come down to simply PvPers and players that don't do anything predominately, something that goes directly against the idea of the mega server and splitting people into the groups of people that player similarly to them, this wont be hard with one group of players. Another important thing to note is that a large amount of people will be coming from an Elder Scrolls background and not from a MMO background, these people will just want to play the single player aspects of the game and will not be interested in PvP - something that the original games do not even offer.
Choice at Endgame
Something less obvious when it comes to why someone enjoys a game or not is about having to choice to do multiple things or just one thing, games that are successful are the ones that offer lots of different things to do once max level has been reached, these things include roleplay, crafting, exploring, additional quests, PvP, dungeons and raids - for a lot of people things like dungeons, crafting and additional quests come under the same category which is raid preparation, these are the same people that would not normally find roleplay, exploring and to a lesser extent PvP enjoyable which means that when it comes down to it this group of players will have nothing to do when it comes to end game and this is no small group, they probably fill up just over 25% of a MMO's population. For the other players who do not fit inside this group and find themselves doing other stuff with their game time, similar patterns can be made and when you look into it, they as well have nothing to do when they want a change from their normal schedule.
Guilds & Community
Speaking for myself and millions of MMOs players out there, I can rightfully say that the only reason we have played MMOs for so long is because of our guilds and the community that they bring, something that is unrivalled by any other genre, spend 1 hour on a MOBA or even a FPS with the 5 or so people you are able to and you will run into multiple flamers, arguments, ragers and all the rest; on the other side of things, I can log into any of the MMOs I have played, be greeted by no less than 20 people, partake in some day to day objectives with strangers and only meet a flamer one time out of ten. The is the case because of way MMOs are set up and the direction in which they channel emotion compared to any other genre, when it comes down to it, playing against other players and losing to them will always bring up negative emotion and in most people bring only that, when you lose to an AI as part of a team the negative emotion you feel is greatly overpowered by wanting to help the other 8-24 people in your raid and overcome the objectives together. The negative emotion is rarely pointed towards another member of the community and when it is there are far more important things to be worrying about, this is the complete other way round when it comes to PvP, it is always someone's fault and you are never get the feeling of being part of a group that holds back the anger, it might take weeks or even months but eventually all this hate, anger and frustration placed on a small amount of people leads to issues and when this is happening amongst lots of groups within the game it ultimately leads to the community we see in MOBA and FPS games and trust me when I say that this is the silent killer. PvP just does not bring the same community values that PvE bring and even if PvP players are not going to use the raid content and the game will be PvP orientated, you need the atmosphere and emotion that a organised community of raiders will bring - the players that will be the ones structuring mature guilds, the ones playing 15 hours a day populating the economy and the ones become the backbone to any successful MMO.
The Facts
Below I will list some MMOs of recent times that were made to be PvP based or have come out with limited/bad PvE content
- Warhammer Online
- Guilds Wars 2
- Star Wars: The Old Republic
- Age of Conan
- Rift
- World of Warcraft
- Lord of the Rings Online
- Aion (to a lesser extent)
Conclusion
I am amongst everybody when I say I do not want to see ESO fail and I am certainly not scared of a bit of change as long as it is the right change but this is not the right change, raiding is something that must be in MMOs for so many different reasons - by not having raiding in the game they are automatically cutting their sales by at least 20% which in turn will bring in less money and less content which all has the snowball effect we have seen so many times in the past. Adventure Zones will not cut it and so far this game is going in every single direction Guild Wars 2 went and just as Guild Wars 2 people will stop playing it and the game will slowly but surely fail just as nearly every other MMO that has put a back seat on PvE content. Raiding is a must for MMOs just as PvP is, without it the community will suffer, the sales will suffer, the players will suffer and 2 years for now we will all be making the same posts on another hopeful up and comers forums.