This might trigger a few people in this topic.
The closest thing we had for actual co-op was Trials, and it was removed 2 years ago. Let's be realistic here, who actually played trials before, and not understand the fact that Warframe playerbase consists of everyone in their own islands spamming 3-4 buttons to progress, just to come out so often and complain how trinity is op. Meanwhile in those 2 years, we never get to experience co-op, because Trials was literally the only game mode that actually provides a "co-op" environment.
In fact, Scarlet Spear is literally a cardboard cutout version of Trials, it requires more than a handful of brain cells, it's very buggy, needs a guide to explain what is going on, and requires other people to somewhat cooperate.
However, just like the cardboard cutout, it's nothing like trials and it's plagued with other problems, like how every enemy was essentially a bullet sponge, and you have to shoot through every bullet sponge to progress (thanks to enemy scaling).
Scarlet Spear is the result of Digital Extremes trying to re-introduce "co-op", and forgot that it's been 2 years since we've done anything co-op related.
Personally, I don't think being a bunch of raging red ants with no queen trying to take down a larger insect in the fastest speed possible is considered "Co-op"
Eidolon hunting was more of "this is a large pointy shooty target that takes more than 30 hits, and we shoot till it's dead"