Definitely not today, no. Don’t think of a server as a single piece of hardware that crashes with too many connections. Those are dead.
Servers are grid/mesh based systems which is why you won’t get crazy load spikes and which is why you can bring new realms online with a few clicks. Think of the entire grid as a HDD. You’re not limited to 1 folder per drive. You can create as many as the storage medium fits. Realms are spread across the grid in each designated time zone with some compartmentalization for safety. You can have (random number) 20 Realms on a single grid. One realm being at max capacity is no different than that exact same number of players spread across all 20 Realms.
The only worry about overpopulation in zones in 2019 is spawns, crowding, client performance due to the number of vertices (meshes) in the game cameras viewport (more meshes needs more CPU/GPU), etc.
You won’t be able to crash a “server” with level 1 Gnome Warriors marching to IF anymore.
If you’re not seeing people in a zone while levelling in retail, more often than not it’s because of phasing. The old world has been given some pretty heavy phasing all throughout. I could complete a questline not seeing but a few people prior, then all of a sudden i complete the questline that was tied to the phase and i’d be in the updated phased content and see more players.
Sharding the launch experience is the smartest, best way to handle the initial rush. There will be enough people playing at launch that the servers will still feel lively.
On top of that, zone chat is not sharded, so there will still be easy ways to group with people, and grouped people will always be able to see eachother (outside of phasing which won’t exist in Classic.)
Didn’t blizzard say that sharding will only happen for a few weeks after launch, and it will only affect starting zones (and maybe zones like Westfall, the barrens etc).
They didn’t give a lot of details but they did make statements that could easily be interpreted as being limited like that. They didn’t even state they’d use it, just that it was on the table.