Server culture matters. That is one of the most important things people are advocating for with Classic.
Let's say the high amount of tourists flooding the game lasts for a month.
SameNameServer 1-5 are each going to develop their own culture. Once they get merged, those unique cultures will be irrevocably altered, especially for those SameNameServers with the lower populations.
How do I know this? Because I experienced it when Blizzard connected the server my characters were on to another one. The entire culture of the server changed. Not for the worse, necessarily. It was just, different.
Sharding is not ideal but I definitely prefer it to suddenly having thousands of people whom I do not know, and have no relationship with, join my server
But Lightforged, we could just create SameNameServer 1-5 but link their chats and auction houses. We could even allow players to group up together! Then there will be one shared culture on the server! My response would be, congrats, you just created a more complicated version of Sharding that also breaks the Design Goal of not allowing cross-server play. It also runs the risk of two servers gaining populations that are large enough to not be able to be merged together, which means you are stuck with SameServerName 1 and SameServerName 2 forever more (good luck trying to change one of the server's names. It will cause a massive backlash).
We don't know how popular Classic will be; what the population of tourists will be compared to the long term player population; we do not know how high or low the population caps will be; we do not know how many people Blizzard intends to be on the server at peak hours. Point is, we have very little hard data to go off of. What we do know is that Blizzard believes the number of tourists coming to Classic WILL be large enough that it will ruin the experience, both for the tourists and the long term players, which is why they want to use Sharding. There are lots of other solutions to the tourist problem but they are just as fraught with other problems as Sharding, while also being more complex and less friendly to long term players and tourists.
I do not want sharding but it is the least bad solution to the Tourist Problem, IMO.