Nowadays, most MMORPGs let you reach maximum level withing days or weeks, especially if you find the fastest leveling alternative.
Games are noticeably speeding up in many ways and this goes for character progression too in many cases. In Aion, as the new expansion arrived in the western regions, after the campaign quest that lets you hit level 66, you will quickly realize that from that point on, you will not be getting 3-4 levels a day.
Leveling in Black Desert is fairly fast up to around level 50 and this can be achieved within a day just by grinding effectively. But from then on, players who cannot stand having to grind for hours just to hit a quarter of a level will quickly drop out from the game. This slow experience gain system seems like something that would only work well for extremely dedicated and hardcore players and perhaps veterans who are used to the leveling systems of the old days.
For common and casual gamers, the faster and easier MMORPGs are probably a lot more appealing and they will most likely choose those instead, which probably hints that slower games are bound to have a smaller player base. This is, of course, only true for NA and EU regions, as Asian gamers seem to be okay with MMORPGs that come with extremely slow leveling and tons of grinding. This is probably why games like Aion still have a huge player base in Korea, as they don’t mind having to play an MMORPG with a difficulty level that is well above the average.
Most MMORPGs come with fast power leveling alternatives that will significantly speed up your character progression, but there are a select few in which you will simply have to put up with the slow experience gain and lack of power leveling opportunities.