Personally I'd say that enjoying the content is extremely relevant. No reward is worth doing content you do not enjoy.
There are plenty of mounts in the game that require far less effort than the hunt achievement mounts. Maybe people should stop and think about why they feel obtaining those 3 mounts is so important hen they'll likely never use those mounts past the first week of getting them.
Sounds like a lot of time wasted that could have been spent doing things that they, I don't know - maybe enjoy?, instead.
People bring more grief on themselves over their obsessive need to collect and/or complete things that amount to nothing the moment they stop playing the game and don't really have any particular value in the game either. Insanity at its finest I suppose.
Did you know that Criterion and Variant aren't dead as content to all players? There are rewards for doing that content. There are players using those rewards and selling them on the marketboard.
The content may be dead to those only concerned about power progression but that's not the entire player base.
Content doesn't have to appeal to every player to be worthwhile and plenty of players repeat content despite no longer getting any rewards they want/need.
Why do they repeat it? Because they enjoy doing it and that is what matters to them in the end. When there are players willing to repeat content despite a lack of rewards, the developers have done something right with the content design.
I wouldn't say the developers made it the most efficient method. Instead I would say that the developers allowed players to turn it into one of the most efficient methods.
A good group of players could spam level 90 dungeons and get Causality almost as quickly. The difference is there is more personal effort and responsibility required when spamming dungeons.
The organized hunt has become the simplest way for most players to get their tomestones and now some are looking to get it changed so there is even less effort and responsibility needed.
It was an issue in the past.
We had overcrowded trains in Stormblood when players were still limited to their homeworlds.
We had overcrowded trains in Shadowbringers because they were always a good way to get tomestones.
The overcrowded trains in Endwalker have been a longer lasting issue due to the relic grind but they haven't been unique to Endwalker.
We will no doubt continue to have overcrowded in Dawntrail because players are used to the convenience in getting their tomestones.
Technically, it is a hunt design problem in that the design make the trains viable and a good source of tomestones for the time spent. If the respawn windows were 2-8 hours instead of 4-6, trains would be stray clean up at best instead of full 12 mark trains most of the time. You'd see a lot more spontaneous kills on sight because who is going to have the patience to wait around for 6 hours for the rest to show up?
If the average player could farm level cap dungeons for their tomestones and get more in the same time it takes a train to clear 12 marks, players would move back to dungeon spam from trains because it would be more efficient. I know that the more highly skilled players can get the tomestones faster via dungeon spam even now but the average player in a group of average players can't.