Honestly, I agree with the sentiment here. While guests are perfectly fine and understandable in-story, and are entirely valid to use at lower levels or for tough quests (or even because you just like having them with you), they don't have a place in an endgame challenge area. The basic reasons that guests are unhealthy in the Inn at the Edge of Time have been stated already by others (RNG and bypassing mechanics, mainly). Inn bosses are absolutely not designed with guests in mind, and while the damage increase is noticeable, it's really not enough of an anti-guests measure. Each Inn fight has its own unique mechanics, and many of them simply cannot deal with guests. Arguing that a simple damage increase across the board is enough to make this fair is disrespecting the uniqueness of each of these bosses.
Naturally, the argument can be made that without guests, the skill floor of challenges is increased, and fewer people can win against them. This is certainly true, and one only needs to look at the history of questions and complaints about the inability to use guests against the Undead Duo. However, this is not a bad thing. In the RPG genre, there are many examples of moderately to extremely difficult bosses - especially optional ones which have little to no bearing on the actual story. Just like in the Inn at the Edge of Time, people who fight these bosses know they're going into a difficult fight that is a cut above the rest of the game. Loss is always a possibility - it might even be the most likely possibility. In these games, players will learn these fights' mechanics, try out strategies, confer with other players, and get used to playing the fight, all in the name of eking out a victory against harsh odds. If a player isn't prepared or doesn't learn the fight or just simply isn't good enough, then they lose. The Inn is not a place for all players. It is a place designed to challenge the 1%, so to speak, who are max (or close to max) leveled, that collect the best gear and seek out the hardest fights. Any argument that implies it's meant to be accessible to everyone, or that its accessibility should be increased, simply falls flat. No one needs the gear from the Inn to play the entire rest of the game that is not the challenge arena. Players get Inn gear for harder Inn challenges. Dragonfable, as a whole, is a very accessible game. Very little of the game is the Inn, and no one is forcing people that aren't ready for the Inn into it.
Another possible argument is simply that people who don't like guests don't have to use them. This is certainly true, and people who don't like them don't use them. However, this does not change the fact that guests are unbalanced and problematic. Issues don't cease to exist if a person decides to ignore them. Now, considering Dragonfable is a single-player game, there may be those who argue that balance is not as important as multiplayer games. Whether or not this is true, arguing that an unbalanced aspect of the game should be left as-is solely because it's a single-player game is incredibly disrespectful to the work that Verlyrus and other devs do to keep the game fun and balanced. There is a long history of buffs and nerfs in Dragonfable, to classes, items, bosses, and other things. Much care is taken in the name of making a fun and fair game. For example, the Chaosweaver class went through no fewer than 9 iterations before the final result was released, because Verlyrus cared deeply about making a fun and balanced class.
But guests are a part of the game, aren't they? Yes, they are, but not every feature of the game should be usable everywhere. The Linus pets can shrink enemies, and many bosses have Shrink resist. Some quests have healing pads, while others have none, making the quest harder. DmK v1 is a part of the game, as well, and it's banned from the Inn - because it's horribly unbalanced and would bypass the mechanics of an incredible amount of fights.
Lately, I've been fighting Pandora EX a lot, as many other people have been as well. It's a beautifully designed fight, both visually and mechanically, and seeing the variety of strategies and optimization is amazing. Participating in conversations about how best to defeat her is some of the most fun I've had recently. Seeing and hearing about people who don't understand her mechanics and don't learn the fight RNG their way through with guests (when the original gauntlet should've banned guests anyway!) hurts me for more than one reason. Using guests means a player is bypassing and ignoring the interesting mechanics of a fight which had a lot of time and effort put into it - not to mention how it cheapens the strengths and weaknesses of the player classes. When I hear a new class has defeated a challenge, I want to know how that class did it - not about how Sir Leon or Artix or Nythera or Mazurek carried a player through the challenge.
Guests do not have a place in the Inn at the Edge of Time. (Also, lore-wise, it makes zero sense to have them there. An extra-dimensional area with a multitude of copies of the hero that the hero can't remember when they leave ... and they bring all their friends with them?)