And yet the market (the users) choose to use Slack over alternatives. If this lost productivity was a problem for the general market, Slack wouldn't have the market share it has.
From what I can see Slack's market share is dwarfed by Teams'[0][1] and similar to Discord's[2]. This is not strictly an argument against your broader point as I believe all these apps are essentially the same in technology stack.
Regardless, it's hard to look at that one feature and determine that conclusion from it. There's a of other pros and cons to be weighed when making any kind of decision like this. I'd suggest hosting, overall UX, and corporate support probably make up far more of the critical success factors for these kinds of apps than their older competitors. The lost productivity and poor performance are the cost of these other features, apparently. Perhaps in the future there will be a new disruptor to the market that will do this same thing but with a fully-native client application that is stabler and more performant than their competitors.
I personally consider IRC (or I guess more precisely, IRC clients like Weechat (Not to be confused with China's WeChat)) a contender, as it provides chat functionality as well as the ability to create channels for themed discussions.
No, it doesn't have emojis or inline multimedia, but IMO it delivers the important part of chat and leaves the unimportant parts for other applications (like opening a hyperlink from IRC chat to see the clever meme someone posted). It also doesn't do video or voice, but there are other applications for that as well. For purely text chat, IRC is a lightweight (and self-hostable FOSS) solution.
