Raids, Log on once or twice a week to complete your weekly run of the same 4 bosses in hopes of a +7 weapon, and try to get as many as you can for your guild or yourself.
most players only log on once or twice a week to complete raids, and occasional shards to test out and use their weapons.
Shards, making shards the new endgame will increase the playercount and activity of the game, if we think about the past when was the highest player count within the previous few years, it was before boss 4 or hard mode citadel. Reason being everyone was logging on daily and for extended periods doing level 15 shards to get their +6 weapons. making more shards will increase the player count by making players log on frequently and for longer periods in time compared to raiding once or twice a week. this will also bring more people back with new fresh content.
TLDR: Making shards the endgame will make more players active and log on more than their one or two raids each week and will keep them online longer farming AND will make more players be able to do the new content in smaller shard groups rather than full parties, all working together for those new items in the higher level of dungeons.