It being fun or not is irrelevant. Manic grinding is necessary, as this is the ONLY source of good xp in the game. Killing yourself to cut the run short is bad because it is fundamentally broken on a game design level, not because it's "not fun". Doing this should never be a practical means to play a game. Ever.
And yes, the end goal of this game (any mmo ever, actually) Is to hit the level cap and progress further in the endgame. Taking your time or rushing to the cap is purely subjective on the individual player and has no relevance to this discussion, which is about progression mechanics. The speed at which the player chooses to progress though a game has no bearing on the activities the game provides for the player.
The problem is you are pretending like there is some other way to level in this game, there isn't. The only way to level effectively is to Grind Ep4 manic solely for the mob xp. This is the only source of consistent progression and gear in this game, there are no other viable alternatives to play, so you will have no choice but to embrace this playstyle. All of this leads back to my op, how this playstyle is unsustainable assuming all players want to forever embrace the current difficulty of manics, and the dropping playerbase in leveling content cutting off the option for those who want to group for whatever reasons need be.
This can be fixed by distributing some of this xp that is earned through mainc grinding into OTHER activities the game could offer, like questing, nomal/hard modes, steel graves, crafting, giving dailies an xp reward, dynamic questing, even PvP, etc. This decreases the absolute reliance on grinding Ep4 manic and increases gameplay variety for the player.
Or decrease overall xp needed to level is another alternative, either one would work.
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So from what I'm understanding, your main points are:
1. Players shouldn't have to use the suicide manic run strategy if they want to level up in an "efficient" manner, because a dungeon is meant to be cleared.
2. However, manic runs are difficult to solo, and you fear that eventually there won't be enough players to consistently pop auto-queue.
3. I'm not sure why you want to redistribute XP from manic mobs into other activities; to promote players focusing on other aspects of the game?
However, looking at your solutions, it seems that your problem has more to do with the fact that players would have to run manic difficulty at all, especially down the road in 6 months or however long. If that is the case, then like I've stated earlier in this post, I believe you're stating something that game publishers are already well aware of, and is a common practice in the industry. I'll use my experience from playing Blade and Soul as an example: when the game was just released, dungeons had all their full intended mechanics. As the player population progressed, and lower levels became a deadzone, dungeons were simplified, and some mechanics which were required to beat the dungeons were even removed so that a player could solo it.
Game publishers are well aware of this trend, and they've been doing it for a long time. There is no need for you to cry out about how this will be the reason the game dies, because they WILL change it as they need to.
Now, to address some of the issues you've been bringing up: to make clearing dungeons more appealing, wouldn't it make more sense to just simply increase the clear exp or give bonus dzenai when you clear a dungeon? To deal with #1 stated above, it's a matter of making the intrinsic rewards for clearing a dungeon higher than the intrinsic rewards for running the dungeon twice without the boss. Currently, the clear exp is relatively trivial, and the only reason anyone would run to clear the dungeon is for the district completion rewards, or for loot drops (uniques, blueprints, etc). So if GameForge were to edit the game so that players were rewarded better for CLEARING the dungeon, they would no longer (or be less likely) to use the suicide run strategy.
My point here is that I believe your approach might be a bit flawed, and some of the suggestions you brought up are not applicable to this particular nature of game. You may have experience playing MMOs, but recognize that there are various genres/subgenres of MMOs. Dungeon crawler MMOs, like SoulWorker and Blade and Soul, operate very differently from open-world type MMOs like Black Desert, Archeage, and an old one from my childhood, Runescape. In dungeon crawler types, practically EVERYTHING is done through dungeons or something similar; quests are the typical "get this from this dungeon", "kill X amount of Y creep in this dungeon", and so forth, and the only method of acquiring necessary items to do other stuff (other than buying them on the market) is through running dungeons and getting them from drops. On the other hand, take Runescape for example, the process to acquire materials for leveling skills vary greatly; some are collection based, some are combat based.
I'll address each of your solutions in the quote I quoted: Moving/improving exp to
Normals/Hard Modes: will most likely be done in the future anyway without your prompt, that or ease the difficulty of manic mode.
Quests: this is fair, and will most likely also be done in the future where they will lessen the exp curve required to reach level cap so a new player can catch up.
Dynamic questing: you'll have to explain more of what you meant by dynamic questing.
PvP: this could lead to severe power/level/gear abuse situations unless a specific PvP mode/stage is created; that rests on the developer of the game Lion Games, and not the publisher.
Crafting: everything that has to do with crafting is pretty much dungeon related anyway. Blueprints are farmed in dungeons, materials are dropped from dungeons. Moving exp to crafting would only make crafting be another gate as it forces players to craft items (more than likely at points where it is not a necessary activity).
Not going to make a TL;DR because I'd want you to read my points and consider them if you were to reply, rather than not addressing some of the rebuttals, but for a summary: I agree with you that this current exp grind method is not sustainable, but I believe it is not of our concern because it will be taken care of. Additionally, I don't agree with some of your solutions that you offered, see above for reasons and my thoughts. Thanks in advance for taking the time to read it.