There’s no guarantee your instance will be around forever, they are all created by random people across the world running their own servers at their own cost. People may get bored/too busy and move on, can’t afford it anymore, etc.
Also no guarantee the same people will run the instance forever, could get sold/given to new admins who ruin it somehow.
Server could get overloaded with too many people/content and not maintained properly resulting in poor quality/speeds.
Lots of reasons you may need to switch instances, would be nice if you didn’t lose 5 years worth of account history and be forced to make a new one.