I'll grant that 28-30 is a steep cliff that offers very little benefit to solo casual players for achieving it. For newer/less-seasoned players I understand how this is a big downer. You've been cruising through the mid game having a high old time with character progression and then WHAMO, there's a much larger wall and character progression stalls.
On the other side, almost every advanced player doesn't even think about exp, they just focus on skilling and ranking from the time they hit the mid 20s [and in some cases earlier]. Legend drives all pursuits. Exp comes so naturally with those pursuits that there is no reason to think about levelling for the sake of levelling. You will hit 1 xp long before you get your skills, ranks, and spells squared away for legend quests.
So you say, why make 28-30 harder? It WILL give some casual players a good sense of accomplishment if they never seek to achieve legend, even if it provides limited to no benefit to each individual character.
I would say that the reason frustration sets in at 28-30 is not because of the noticeable slowing of progression. It is because at that point you are still trying to play the "mid level" game of regular character progression, when your character has become ready for the "end game" game. Unfortunately, fun in the end game except for the most introverted of us requires making friends to hang out with while you are in zones, building a stronghold, etc. I can only imagine that being very difficult, since most of the people who still hang around have probably 10-20 year histories on the game and feel like they've already made their friends.
The only real question is should the game's leaders, i.e. Kam, make it so that people who really enjoy the mid level game (read "casual solo" with rapid progression up to level 28) have something to do for a longer period of time. [Right now it probably takes 24 hrs of game time to finish the mid level content for almost all comers. If you're like me it probably can be done in 10 hrs.]
TLDR: Sure I stopped playing this wipe when I soloed up to level 29 and stopped making progress at a fast rate. However, the reason I stopped playing this wipe at level 29, had much less to do with progression. It just had to do with the fact that given the way I was having to play (20-30 minute bursts of free time) I couldn't really play with my friends in any meaningful way and that is how the end game is meant to be played--my life doesn't give me the time to zone with others.