Why bother getting gear when it's just going to be nerfed when next expac drops and you can buy vendor gear better than it.
Content vs Rewards is a more complicated manner than "Item rewards = only reason to do content" or "Item rewards = incentive to do content more than once"
It's about finding the right balance. Content that is fun enough to keep people interested, and a reward that entices people to enter the content in the first place.
You can go more heavily towards the reward and force people through not fun content purely for the reward (Though, this breeds resentment). Or you can go more heavily towards fun content and have something exist purely based on people wanting to waste time there (This tends to make content less populated)
Also, the logic that Eureka needs to have the Relic reward as the carrot on the stick to get people to go through it is asinine, especially when by your own words "You get full gear sets, mounts, minions, glamour, hairstyles and furniture and lots more."
Literally, they could have "Designed" anything to be the reward for running through Eureka. Only this would have meant designing a Relic weapon AND an actual proper reward for Eureka (Though, full gear sets without having to farm duties is a significant draw for some people)
"Why grind though?" is the common question in response. Often implying that the heavy amount of grinding necessary for obtaining the Relic in Eureka is:
1) Necessary to have in such large amounts
2) Couldn't be directed towards something else (For example, they could have made leveling focus more on doing FATEs and Chaining as opposed to grinding these things for Crystals/Light)
That still doesn't mean it's "Designed" around it. Only that its implementation coincided with the release of it.
Given that Eureka was delayed in the expansion compared to the previous 2 Relics, this isn't that much different.
Like, if when they released Eureka they just made The Lochs FATEs give crystals while using your Antiquated 70 weapon it would have been the same thing.
To say nothing about how you can equally argue that much of Eureka is designed for leveling to get through the Eureka story (That doesn't involve the Relic).
You can go through Pagos without talking to Gerolt and getting the ability to farm for Light.
You can get Logos without having the Pyros weapon.
You can go through BA without having a Relic weapon.