Problem is you have to deal with the realities of human nature being what they are and Blizzard’s inherent promise that “your player character is safe with us” which makes Server Merging highly problematic. Because server merges either force a character name change(your character isn’t so safe after all), or you get the Server-Surname thing going on.
So that said, it is “easier” to deal with a Overpopulated Realm than it is to deal with a dozen underpopulated ones. Which takes us back to “you shard the starting areas” and hope most of the Retail tourists will quit before they hit level 10, or level 20 at the latest. After which “normal” server operations can handle the rest of the load.
If they’re still around by level 30+ that’s when you start capping realm populations, introduce server queues, and offer free migrations to new/other servers to “spread the load around.”
But we’re in broken record territory here. Your desire for classic in the most strictly literal way possible is well documented. You seem to be hide-bound and determined for players to see people running around with server-surnames 6 months after Classic goes live because “the tourists” were not handled at all because “Vanilla WoW didn’t have a tourist problem large enough to warrant designing for it.”
Time will tell, we’ll see what Blizzard decides to do, which side “wins” that round of the argument, and ultimately which side was right in the longer run.
In some respects, it’s potentially kind of moot anyhow, as the initial servers are likely to be safe either way. I expect the “stable population” is likely to be larger than what Blizzard is likely to initially provision for. So sharding will likely be more of a tool to allow people to play the game rather than sit in queue while Blizzard goes about adding additional servers for them to transfer over to.