While I agree they're over too fast, some of the holiday events in other games go to far to the opposite extreme and end up very grindy with a lot of RNG involved. They also don't always release new rewards so when you've already gotten everything, there's no reason to do them the following year. MMORPGs in general need to find a better middle ground for seasonal events.
You're not talking about leveling. You're talking about the MSQ. There's a lot of different content to choose from when it comes to leveling (provided you've advanced far enough in the MSQ to have unlocked it). This being a story driven game, the formula isn't likely to change.
Strange how I've heard the opposite about normal dungeons from content creators that play both FFXIV and WoW. They find normal WoW dungeons excessively easy compared to FFXIV. It's only once they reach the middle keys in M+ that they start finding the WoW dungeons more challenging. And by the time you get to those, you're done leveling and focused on the end game gear grind.
Anything can happen was true of many of the original Vanilla WoW dungeons but they're just a memory now. Blizzard ended up streamlining them and later dungeons (unless they changed dungeon design since I quit at the end of Legion). Even when I was still playing, you knew what was going to happen for boss mechanics and players had figured out a most efficient route (including what trash had to be pulled and what could be skipped). They weren't anymore engaging than FFXIV dungeons by the time I quit.
It's doubtful that anything here is going to change substantially when it the game appears to be mostly meeting expectations in the JP player base. It is a JP game, not a western game after all. My advice will always be to seek out the game a provides what you enjoy instead of expecting a game to change to please you. Paying for and playing games that aren't enjoyed sends the wrong message to the gaming industry about what games players want to be seeing on the market.
Imagine the lag and other issues if the open world zones were packed with content so huge amounts of players were always congregated in them. What is common at hunt trains now would be a minor irritation in comparison.
That's why MMORPGs began turning to instanced content instead of having everything open world. The game would literally (for a change) become unplayable for many players and that would cut deep into the revenue stream for the gaming corporations as those players quit.
It's the technology available and affordable to the average consumer that holds things back for MMORPGs. WoW really isn't any better when it comes to the amount of content available in open world zones, they just do it differently.