You can pay for Discord. Admittedly, I do. It's not that Discord is perfect, I have a lot of personal gripes with it. But it's still significantly better than where I came from (Skype) and I use it a lot so it seems fair enough. Discord Nitro also thankfully pivoted from being a games service and the features it does provide are nice to have. (Larger file uploads, better stream quality, cross-server emoji.)
Is it? $15-30/head sounds like a lot, but these are employees you're paying 30k+ to in salary alone, if the addition of slack makes them 1% more efficient per month, that blows past the $15/head. Forget the tech companies with employees that easily crest 200-300k in costs after insurance and other benefits.
There's a lot of audits and regulation, in addition to tighter security, that Slack needs to prove to its enterprise customers that they can trust their employees blasting confidential information on it every day of every year.
You are comparing efficiency like in "with Slack" and "without Slack", but in fact it is a "with Slack" vs "with another messenger". On a previous job we used Telegram for work communication. It has bots and stuff, and it's blazing fast. And it's free.
Nitro is for individual accounts and provides features for you as a user, boost is for the server and provides features for every user of the server.
Maxing out a server takes 30 boosts but the level 3 perks seem pretty… thin on the ground:
- +100 emoji (from 150 to 200)
- 384Kbps audio (from 256)
- 100MB uploads (from 50)
- custom URLs
Only the third one is somewhat useful, but 15 boosts for that doesn't really seems worth it.
As to price, a boost is $5 so a level 3 server is indeed $150 (level 2 is half), however Nitro ($10) provides 2 boosts and 30% off all boost purchases, meaning you can max out a server for $108, or 55.5 for a level 2. Nitro classic is only $5 and also provides 30% off of boosts, but doesn't include the free boosts, so it comes out at $110 to get a level 3 server on your own.
This describes how I've felt about Discord for years.