If it wasn't made to cater to the majority of the fanbase nowhere near as much money would have been put into it as there has been. They wouldn't have stuck the relic /and/ job glamour on top of mounts, emotes, and more if they didn't intend for this to be a huge success for the majority of the fanbase. With the time, care, and money being put into it you really can't call it a niche system that was only made to satisfy some to the detriment of others. There are simply too many cattle prods and carrots on a stick trying to entice the playerbase into the content for that to be the case.
Contrast that with Ultimate and even Heaven on High, both of which have highly reused assets with a couple minor tweaks. Those two pieces of content are clearly not meant for everyone and the reused and reskinned nature of both show that in spades, though more for Ultimate than HoH. They're there as options to supplement the rest of the gameplay. There's barely any new system at work. Hardly any new monsters. HoH straight up uses PoTD's system with barely any tweaks, and thus was likely very low cost to make.
Eureka on the other hand had to be built from the ground up. As it uses a fair few brand new model and texture assets, on top of entirely new battle systems and leveling systems that are separate from and need to be balanced against the rest of the game. Instead of small little, copy and pasted boxed zones or a single boss arena we're given two huge sweeping maps to run around in. New magic systems and ways to gain items. I could go on but the ultimate point is that all of this costs a /lot/ of money. Far more than any sane developer would put into a piece of content that wasn't meant to appeal to the majority of players. The issue here however is that what they thought the players should want and what the vast majority actually want are two entirely different things. The same thing happened twice with Diadem and instead of recognizing the lost costs involved and revamping future pieces of Eureka to reflect the currently niche interest, they keep doubling down and trying to fix the already broken system.
I don't think it's unreasonable to wish all that money and time went towards other things. There are ways to give the people who enjoy Eureka a chance to play something like it without sacrificing content for the rest of the game because the developers are convinced that the /fifth/ time will be the charm.