Let's see....
1) Bozja feels much more alive than Eureka does due to the NPCs alone. The character interactions during skirmishes between some of the blades & the 4th legion are pretty great, it gives the battlefield an actual spark of life to the ambiance.
2) in-line with the above, the collections was an amazing addition that made me enjoy Bozja far more than Eureka story-wise with a much deeper, fleshed out lore, character interactions and story.
3) While the duels have their current issues (which can all be gamed quite easily with patience, thanks to dead instances giving me 2-3 duels unmatched in one lockout due to soloing the pre-CE's) The actual fights are infinitely more fun than any NM ever was in Eureka, to the point half the point of my dueling now is just to enjoy them. You know, fun. One of the primary reasons people play games.
And side note: Duels will actually be ultra easy to get into once Southern front dies out. All pre-CE's can be solo'd currently (echo + haste + Bozjan's potential future equivalent to elemental boost making them even easier), so Duels will be easily accessible.
4) Bozja actually makes you interact with it. I dunno why you listed 'harder to afk' as a negative; it's hilariously bad design if one of the optimal ways to level up is to not play the game outside of very short windows. It's why I opted to mob grind in Eureka whenever NMs were down. I wanted to play the game, not be completely bored waiting like I was in Eureka. Bozja is already infinitely better on that fact alone since the CE's are far more engaging than the NMs were outside of the boss ones.
5) You can upgrade items outside of Castrum by doing skirmishes & CE's for coins. It may be slow, but contrast Eureka, where BA is literally the only source of Eureka fragments, meaning if you don't do it, you literally can't progress. Bozja is empirically better in that regard.
I had more fun going to rank 15 than any Eureka instance outside of Hydatos. And that was purely because of BA, not the NMs bit. Complain all you like, but Bozja empirically does several things better than Eureka did, whether you see it or not.
When people raised concerns about Eureka being impossible down the line due to being unable to solo Pazuzu/boss NMs, they added options to make sure catch-up players could proceed in the event the instances died out, no doubt they'll add something to Castrum to make it future-proofed when the time comes. IMO, All it'll take to future proof castrum is to stick the resistance blades in there if there's not enough players and give them jacked stats to mow down the bosses.