Well there's different market segments. Me on one market segment, I'm the guy (nice to meet you) basically the way a sow has a runt, on the off chance she can feed it, that's the same basis companies hire me. And only very fucked companies.
The basic reason is because I got tortured with an amnestic lobotomy. I had a very marketable curriculum until that point, and after that if for instance I was asked where I worked during April 2009, I would ask "are we talking about the Gregorian Calendar, that shit?"
So for me you have to provide amazing shit for me to subscribe to anything. And it can be done. Not often. I can't have 7000 apps draining me $5 a month. That is why I identified a natural limit to the number of web-app subscription-model startups there could be, the first limit was remembering all the credentials to use them, which 1Password and similar surpassed. One user here talked about having 800 web app username-password credentials on his 1Password, obviously doesn't pay any scratch for most of them. Veteran of the freemium model, like I became a veteran of the psych ward in my remarkably successful struggle under torture, including not caving to the torture.
I talk about this at https://news.ycombinator.com/item?id=32620302
So my segment of the market, I'm gone. Neither feature you mentioned seems...wantable. And that goes for most web apps, like I predict their business model and then operate on that basis. So for instance most VPN's are, as a business, honeypots. Like yeah you can steal from the labels who steal music from the artists, that you can do, but for tricky nitty gritty, nah. It's just too obvious, it's just too instantaneous an erection the founder gets thinking of cutting deals with Kuwaiti Intelligence during the commercial of them going on TV saying "We never have and never will cut a deal with Kuwaiti Intelligence."
So with the paying users it's like ice on land, they need the ice on the sea in order to hold them back from falling into the sea. Non-paying users provide a lot of value to Slack, in that for instance you can do a trial release on them to debug your product for the paying users. For example.
"Slack just really sucks at b2b marketing" well compared to Microsoft yes. Slack sucks at marketing compared to Microsoft, for B2B. I buy that. The guy Butterfield (Stuart? Something) was busy getting on magazine covers like it is still the previous millennium, which for businesses loses credibility.
So I went to slack.com and tried finding what they charged, they wanted me to see all their flair first. So I went to the URL which said https://slack.com/features and replaced "features" with "pricing" and there it was, the chart with the numbers they want. So they want $8.00 USD a month for their product, and assumptively propose you buy it for the year at a pro rata price of $6.67 as the default. I looked for their asterisk at the bottom of the page, but it wasn't there. No this was a javascript asterkisk, no relation to asterisks in eg legal document. So $8.00 a month, per user.
So that right there is a turn-off to an informed buyer, which B2B is. There's typically a designated person in the office for this sort of purchase and she herds the company's users through the spam, the spear-fishing, the shitty deals like this one, every everything. Apparently Teams is in fact the better product, and that might be because the terms of service aren't demonic (though I vaguely recall them yes being demonic which is why I couldn't use it, but then from the dodginess in revealing the price I divine Slack's terms are demonic too).
But whatever. In for a penny, in for a pound, sell your soul, no reason to pray. Microsoft already has telemetry on like blood type in the first 18 minutes so it's not an additional leak to work with them.
And that's saying as someone conflicted about the company, I like the way they didn't fuck up github for instance. Didn't nix it. Like they never even bought it. It works better now, in fact, I can tell it has more integrity and reliability. Backend stuff.