I don't think itll be as bad as people think it will be
ShadowBringer also had way more people than Stormblood,Stormblood only messed up because of 2 story instances at the start of the expansion so just about everyone was bottle necking the game,Shadowbringer didnt have any story instance till after the first dungeon because they learned their mistake.
we will have long log in ques and maybe a dungeon instance crash but i dont expect anything like HW or SB