Fixed servers work if the population is more or less stable. When you have rapid in- or outflux, then it doesn't. chances are servers will be 'full' in no time and so playing with your friends on the same realm will require a lot of synchronization and even then some luck of making it all in. A friend coming in even an hour later will probably not be able to join you. Then if predictions are right and a lot of players will either abandon or just play during lul times in the 'other 'WoW, you are faced with your 'active' population spread thin over hundreds of servers. Those calling for 'fixed servers with 3000 population cap' just haven't thought this through.
Sharding, while I get that from a 'purist' perspective this is not desirable, is probably the best option. In your shading strategy, you can still try to optimize for being more likely to see the same players over time.
Btw: I think they will use an 'invite' system for launch. That way they can control ramp-up and placement better.