Let’s take HexChat (https://hexchat.github.io/) for example - it does most of what Slack does, plus has advanced features not available on Slack like a Lua, Python & Perl plug-in API. It used less than 100MB or RAM after running for days on a Linux machine.
Slack is usually well into the GB of RAM after running for a few days.
The keyword here is fancy. Hexchat is an IRC client, sure, but it is minimalist. That would be like comparing a text file to a PDF and asking why the PDF needs so much more RAM when they're basically the same.
I feel like you're mutating the argument from "electron uses 10x RAM compared to native apps" into "an IRC client doesn't need to use so much RAM"
The proper comparison would be either a full, feature complete native Slack client with all the same bells and whistles that runs under 100mb, OR an electron port of hexchat with its text only interface that uses 1GB+
Not at this scale though. Office 2013 uses maybe 2x or 3x the amount of RAM compared to Office 2003.
A chat client like Slack routinely uses up to 1GB while a native equivalent would use 100MB or less. We’re talking about a 10x increase here - that’s unacceptable.