The change shouldn’t affect leveling experience since you can easily level without the help of the personal story. As long as you do events and 100% the map, it won’t matter. I for example simply follow the personal story and 100% whatever zone it takes me to, which gets me through ~20%-30% of the game before reaching 80. People that try to level through traditional questing however are going to have a bad time.
With the personal story being delivered in chapters, I suspect it’s going to create an odd leveling experience. Now you’ll have to go backwards to complete the personal story since what was once 1-10 is now 10. It’s also going to cause level jumps, just like crafting, since the entire chapter will probably be worth something like 5 levels, which you are now encouraged to complete all at once rather than as part of your journey. For those that had problems with leveling to begin with, they’ll likely struggle to make gw2 gold to level 10, at which point the entire chapter will carry them to the next zone, causing a feeling of uphill-downhill-uphill-etc leveling.
Also, the personal story being moved to level 10 has nothing to do with key farming since they moved the key to the level 20. This change is in response to complaints about the flow of the personal story. Back at launch for example, the level gaps between steps was a common complaint from people trying to quest their way through the game, since they felt they needed to grind experience to catch up.