Electron or not, you need a web browser to display content. Unless you want to replicate a browser's layout engine, poorly. History and the rich interface are the main appeals of Slack.
I guess you could create a QT app or something and embed a web widget inside. Not sure how much that would help.
You can replicate it with native widgets. Heck, you can draw everything yourself if you want. But why bother, if you already have a perfectly good way to display that sort of content?
The problem is not "lazy developers using Electron". The problem may actually be lazy developers, but not because of Electron alone.
To me it's like sending a word doc attachment because you want to use bold type in a mail, and you already know how to use word.
Slack could easily use native widgets, without embedding any HTML.
For instance GTK apps in the earlies 2000s were easily displaying content as rich.
Point is, native or not native, this should not matter for a well optimized code. We are not talking about a AAA game here, it's just a chat app. It should not be burning cycles when inactive.
Rich interface doesn't mean HTML and CSS. Have you seen good iOS apps? Their interface is just as rich and yet it's pure Swift/Objective-C compiled down to native code.