They never were absent,it's not like they all enlisted at the same time,individual members still had activities

This is the case for the vast majority of boygroups, and for a lot of them, that doesn't prove to be enough to stay afloat. Long hiatus with no group comebacks = popularity takes hit, always.

Cube knew about him being investigated

Nah, doesn't make sense. During Show Your Love promotions, the 4U members wouldn't shut up about the upcoming OT7 reunion, making plans and promises, giving Ilhoon (and the rest of the military line) shoutouts and tagging him on photos he wasn't even in. And all of that came back to bite them when the news broke.

If they knew he wasn't coming back, they wouldn't have gone that far out of their way to include him in everything. It was very visible from the members' words and actions at the time that it was a big shock for them.

dude had managers,the rest of the group he referred to as family and closest friends that all worked in the same company

And this is just naive. No manager follows their artist around all the time, they are human beings with set working hours. And we have no way of knowing how close the members of any given group actually are irl, and how much of it is fanservice. Yes, BtoB have good chemistry as a group, and seem to genuinely be friends, but it doesn't mean that they don't have their separate lives and friend groups, spend a lot of time together outside of schedules and have no secrets from each other.

the way it was handled was so easy and i wish they dropped Soojin like they dropped him

The situations were very different. Ilhoon was pretty much already proven guilty by the time the news broke. It was straight-up impossible for him to come back from this, so he withdrew immediately. Soojin, in contrast, insisted that she was innocent, plus her scandal was much less severe to begin with, so there was a good chance of her actually staying in gidle. Cube tried to use that chance (until the stupid "I'll leave the group if I'm guilty"-statement ruined it), and that took time.

I'm not talking about sales only,i'm fairly certain that their charting on Melon won't be even close to Hwaa after this

Well, in BtoB's case both Show And Prove and Outsider charted at around the same levels as Show Your Love, so my comparison and my point still stand.