They don’t function quite in the same manner though. Sharding is meant to create instances within given zones. So in this manner you could say it is similar to Layering. What else is similar is that they can apply a player cap with them. The difference comes in when you change zones. In this case, Sharding would function like Layering with Eastern Kingdoms and Kalimdor except that Sharding does it with every zone you transfer to. Thus you could end up seeing players shuffling far more often in Sharding compared to Layering which people are using as an example as to why Layering is “immersion breaking”. Layering also puts certain actions in place that can allow players to hop into the different layers thus allowing a wider range of interaction with players in the zones.
So basically it will be a high population server that will keep all zones sharded at all times until there is a need to remove it or phase 2. In terms of economy, this is about the same affect as Layering, though with certain aspects being questionable based on the player caps they put in place per zone. Like an example for exploitation would be running as many players into a zone and thus creating more instances of Devilsaurs, Black Lotus, and so on. Any zones that are not sharded would then face potential issues of excessive competition to resource ratio.
The question was more based on server population in all honesty, so perhaps we can ignore this since you have applied the route of “less servers, high pop”. The response is basically the same.
The “TL:DR” is basically taking into consideration of the player caps within given zones if Sharding were the tool in place. Also figuring out how you could limit the immersion breaking element of having players jumping in and out of your world more frequently. Balance within the economy also becomes a concern.