Is Bozja or Eureka good because of participation? No. Amount of players in content does not equal fun or quality nor if the players want said content. Here is the context of the numbers missing. Also quality does not equal fun. It is often quality content (high production value, good writing, fun mechnic, etc.) that is fun but this does not mean that those things are a necessity to make a game or part of it fun. These numbers are useless because we dont have any idea what measurments, source or even method and so forth is used. Numbers without context are only used to gaslight or even outright lie to people. Example: We have now 10 % green energy.... in a closed environment and not the normal grid, only usable to certain people, not compared to the whole country because that would be less then 1%, not explained why this energy is even green - and with context this number is not so good anymore.
My point is Eureka/Bozja and even Island Sanctuary had rewards only obtainable through said content. So i say Eureka/Bozja was not loved or popular in regards of "we want that mechanic or playstyle" but "yeah FINALLY REWARDS!!!". That's where the problem lies: SE is very, very, very bad at giving rewards and how to structure them. Bozja wothout the relic and with a simple glow weapon on EX level and this content would be DEAD. And that for a good reason or reasons for that all matters. Eureka and Bozja have accessibility problems and difficulty spikes and last but not least a punishment mechanic that is there for the sake of it. There is no "risk and reward" there is simply do not die or get kupo-ed. Thats it. And that is bad game design. Next is repetition because certain things are only obtainable through 1 activity. Then we have obscure conditions for rewards/events without proper rewards. All these things as singular problems are not that big but together we have a very bad situation.
Is Eureka/Bozja a good idea ? Yes, but horrible executed.
SE has no clue about rewards or the structure of it.
Also gating a famous and unique item type behind it and saying "oh look soo many players" is intellectually dishonest and simply untrue.
The only reason we have so high numbers for Eureka and Bozja is this weapon. Take it off and many would not even touch that with a damn pole.
It could be good but SE does nothing with feedback. Same goes for Island Sanctuary - could be good but SE does not care.
At the moment i have the feeling we are at "Mama knows best"-phase and at least they hear our feedback but decide it is not good but we are closing in to Bethesda level of "gamers are dumb, ignore them".