I really think they should do the opposite.

All zones within older expansion should have a third mount speed that you can earn. Maybe add some sort of combat buff people can earn that's active during FATEs in older zones too.

Think about it this way. There's a lot of zones in this game at this point. People are spread out a lot. If you want to see more players crowding up different zones of the game, then it benefits you to enable players to quickly conclude their business in older zones so they spend more time in newer ones. Essentially funnel everyone in to the zones within the last expansion.