Only developers and very tech-savvy people bitch about Slack. Your average person working in Sales, HR, or operations cares not for the memory footprint of Slack.
It's also not nearly as bad as people make it out to be. Yeah, it's slower than mIRC, fair enough. But IRC is dead to almost everyone and Slack took over, so it is what it is.
Maybe the circle I am in simply has great computers. But who knows.
Software used to be both fast and "feature rich" (relatively speaking), designed so users could "grow" and learn increasingly more of it. Now almost everything seems to have idiotically dumbed-down "flat" UIs that look simpler yet still manage to consume more resources than before.
The new Skype is a great example. Compare to something like this: https://www.microsip.org/ Yes, I know they're different protocols, but the point is that IM and audio/video call functionality doesn't need to take as much resources as Skype does.
I'm sick of sluggish "native" apps on my i7 PC with fast SSDs and 32GB of RAM. I don't even want to imagine how those apps perform on low-end devices.