All software engineering choices are compromised choices made from carefully considered trade offs. To argue they made the wrong choice, you have to show they made the wrong trade offs. I understand you mean the user experience is still not ideal, but when would it ever be? When will performance or RAM usage be "ideal"? What does that mean? Should it compute in 0ms and use 0% RAM? There is no right ideal, or true ideal. The idea of an ideal is itself a choice of trade offs. Your ideal could be that you absolutely need under 20ms response times on all UX actions, while never exceeding 100mb RAM. And that could be ideal for you, but might still not be the right ideal for me, hoping to run it on a modded gameboy hardware. And if that was the case, can you compromise on feature? If not, and its not currently technically possible to achieve your performance ideal and have parity of features, then what? Is the right choice then to not release a desktop client for slack at all?

So if you know a way to achieve your ideal, then you can propose that alternative, and you can say, here is how, without much more cost of development, you can achieve X, Y, X improvement on performance while keeping all features. But, if you do not know, and have no known examples, it could indicate that it is not currently technically known how, and thus impossible with the current hardware and software knowledge at hand.

I'd also say that in the case of Slack, their ideal is probably defined more in terms of profits. So it could be said to be the right choice, simply because of how popular their service is and has remained, even though they picked Electron for their desktop client.