Given SoM is 12 months, players will be discarding these characters more quickly, and SoM might not even go beyond Vanilla WoW and just flat out end in 12 months. A lot of people won’t find the time investment to get a character to 60 at blizz-like rates worth it, while plenty others wouldn’t mind since they like the leveling a lot.
They should add an NPC in the starting zones for people to choose their xp rate so people who really like the 1-60 questing to get their normal leveling experience, while SoM players get their characters more quickly. Maybe it has an option for 1x, 2x, 3x, or anything between. Both sides get what they want.