They all made the right decision, as it folded only a couple months later.
Even within organizations there are critically important employees.
So, no, it doesn't make sense.
Slack contains a world of sensitive corporate information.
Oddly though I don't recall any major email company losing gazillions of emails to a hacker either, but I could be wrong.
It's an existential risk for Slack, but not unlike most other SaaS companies as well, so it's just part of the cost of doing business a per the industry.
"Funded businesses die all the time. Google cancels products all the time. companies are acquihired and products end of lifed."
So it makes perfect sense to avoid Google offerings (especially consumer side ones, which Google kills whenever it wants) and avoid "funded businesses" and newcomers that in danger of being acquihired and EOLifed.
Letting the careless early adopt those kinds of services, and using only mature ones (that have proven their longevity and aren't going anywhere even if acquired) in your company, makes sense.
Further, there is much higher trust that Slack would give a reasonable notification period of they decided to stop existing, compared with a single dev's project.
Otherwise you're a jumped-up contractor with a hobby project. And you're not going to be taken seriously as a business partner.
I learned this the hard way in the early 00s. It's one of the fundamental marketing elements in B2B, and it has nothing to do with the quality of the product, its features, its ability to solve problems for a convincing price, and anything rational.
You can even say that anything above the very lowest levels of corporate B2B marketing - excluding the trades and very small mom & pop SMEs, who are a separate market - is about projecting business charisma and potency, not so much about solving actual problems with any efficiency or effectiveness, and certainly not about solving them at an affordable cost.
It's entirely tribal. And if you're a one-person shop you're an outsider - effectively just a potential employee or remote temp - and not part of the tribe.
IMO no amount of marketing and validation can overcome this bias in B2B. You're just not going to be considered a worthy equal, no matter how you spin it, or how good your offering is.