They didn't know if that would actually work, and indeed, individuals are added to the collective everyday without issue, so not sure why Hugh would be different.
The Borg assimilating countless species and subjecting individuals to the will of a singular entity was something that was introduced in FC. When a drone was freed it wasn't a big deal for the hive at all.
In TNG on the other side, there was no individuality (not even as a concept!) in the collective at all. So when a single borg (not using the term drone here for a reason) developed individual thoughts, it had to have a major impact on the whole collective...
Physically, there's not much to be done about that. I mean, Glenn Corbett was kind of dead at the time.
Mentally? It could make perfect sense. When you're living in a post-war society, and people you love are dying and you're losing hope, you might get raging drunk, regardless of your good intentions. It's not difficult to say that after meeting the Vulcans, Cochrane cleaned up and saw a new outlook for humanity.
I think James Cromwell did an excellent job blending the hopeless drunk, the hopeful scientist, and the cynical asshole all into one character. FC isn't my favorite movie, but James isn't at fault for his performance, or for any deficits regarding the film, IMO.
I wouldn't make so much of it too, if some fans hadn't brought up the issue that the actor playing Maddox in Picard doesn't resemble Brian Brophy and criticized the show for it.
So what is different in this case? Even if Corbett is dead, they could have taken an actor that looked more like him.
Because it introduces the amazing E, nice new uniforms, the Defiant and the Borg at movie quality, perhaps the best soundtrack of them all, 24th century spacesuits, walking on the hull, great new rifles, the most important moment of Earth's future history, who the first aliens were, how the first warp flight happened, essentially the beginning of it all.
He overcame part of it in Family, then overcame another part of it in I Borg, then overcame another part of it in FC, and still had enough left to overcome in Impossible Box. Hugh led to a group of 'rogue' Borg getting separated from the collective, not the whole collective collapsing. Was that ever said in Descent?
Didn't Cochrane say in Metamorphosis that the companion made him younger and healthier? Perhaps on the other side of the planet, things were still in the 'postatomic horror', while things in Montana were kinda ok in comparison. Why would the whole planet be in the same state? The Q trial probably recreated some ECON trial, and it seems the ECON lost the war.
"Descent" made it seem that the whole Borg Collective was destroyed or at least crippled by Hugh's return.
From "Descent, Part 2":
""HUGH: You gave me a sense of individuality, changed me, then sent me back to the Collective. You must have known that change would be passed on to others.
RIKER: We considered it. We knew it was a possibility.
HUGH: Then you made it possible for Lore to dominate us.
WORF: I cannot accept that. Lore is only one. The Borg could have stopped him.
HUGH: You don't know the condition we were in when he found us. Before my experience on the Enterprise, the Borg were a single-minded Collective. The voices in our heads were smooth and flowing. But after I returned, those voices began to change. They became uneven, discordant. For the first time, individual Borg had differing ideas about how to proceed. We couldn't function. Some Borg fought each other. Others simply shut themselves down. Many starved to death.
RIKER: And then Lore came along.
HUGH: You probably can't imagine what it is like to be so lost and frightened that you will listen to any voice which promises change.
WORF: Even if that voice insists on controlling you.
HUGH: That's what we wanted. Someone to show us the way out of confusion. Lore promised clarity and purpose. In the beginning, he seemed like a saviour. The promise of becoming a superior race, of becoming fully artificial was compelling. We gladly did everything he asked of us. But after a while, it became clear that Lore had no idea how to keep his promise. That's when he began talking about the need for us to make sacrifices. Before we realised it, this was the result.
RIKER: What happened to them?
HUGH: Lore began to experiment, trying to re-make us in his image. This is the result of my encounter with the Enterprise, Commander. So you can see I don't particularly welcome your presence here."
