If this was the case we wouldn't see so many Electron apps. You just don't perceive enough of the upsides to see why the decisions are being made.
So "the downsides of Electron outweigh the downsides of native apps" when you're someone who patrols the task manager looking for things to be upset about. Your chat client - one of half a dozen applications you realistically have open - using 400MB of RAM on your 32GB workstation does not have a meaningful impact on your workflow. It's just offensive.
(I'm not against web APIs as a platform for desktop apps, but I'm happier using Webkit wrappers and I think this would be improved, cross-platform, if Chrome could effectively host the apps that currently ship with their own instance of Electron or CEF)
However, if your other applications are a terminal, other efficient text editor (e.g. Sublime), and other generally respectful apps, I've found having Slack open is the difference between having enough battery life to work all day, and not.
Even with the performance improved, the "stuff the whole app in one web view" implementation is still showing. Clearly this isn't an Electron limitation; VS Code handles multiple windows just as well as any other text editor.
Slack's desktop client is basically "It's a web browser except worse and with the URL bar hidden." Since chat requires an internet connection and it's not like Slack's desktop version launches particularly fast, I'm not sure that it does anything better than using a browser.
We'll see if they support multiple instances on iOS 13. If they do, maybe we can get a Mac version based on that for something closer to native desktop behavior.
I don't see how it could ever be worse than the webpage. And it gives you nice perks like notification badge integration, which for me personally is essential.
I usually don't need to do that, but when I do it's annoying that the desktop client prevents it.
It's fine that my phone doesn't let me open two text conversations at once since that's all the space it has, but on a computer with a 24+ inch screen and multiple virtual desktop spaces with different sets of tasks, it's a pointless limitation.
The upside that keeps getting paraded around is that it's a huge boon for product velocity. But that's just objectively false because Slack-the-product hasn't meaningfully changed in years.