My understanding of the reason they’re likely to shard servers at launch is the following:
Instead of launching Classic with an absurd number of servers to deal with the incredibly large influx of players at launch, they’re going with a more limited number of servers and piling on many more players than they otherwise would, in order to ensure a healthy community arises and is allowed to flourish after the initial “tourism” wave dies down.
Would you rather they launch with five times as many servers, and three months down the road merge them? Would you like to see four other players with your name and various server identifiers attached to them? Eg: Galdor-Maelstrom, Galdor-Stormrage; Galdor-Thrall, Galdor-Sargeras.