Royals and Chiefs are both incredibly stupid for looking to move away from their current locations
First, they're probably not actually looking to move away; they're looking for ammunition to use to hold the KC market hostage to give them what they want (new stadiums).
Second, fan support is incredibly overrated in this era of professional sports. Just look at the LA Rams - they probably have fewer fans now in LA than they did when they were in St. Louis, certainly in fan attendance. Hell - they had a home field disadvantage in the NFC Championship game when they played the 49ers. But they make a hell of a lot more in revenue in LA than they did in STL. And to a team owner, what do you think they want more - more fans in the stands, or more money in their bank account, even if that comes at the expense of fans in the stands?