There are lots of people building things on proprietary platforms all the time. Why single out FB and Slack? AWS is proprietary. Twilio is proprietary. It's not as if it's hard to find examples.
And so what if some small team implements an auto responder bot and gets a couple hundred thousand in investment?
FB and Slack are platforms in which you write directly for it and it cannot be used or moved elsewhere.
AWS provides many things but, unless you're directly integrating with them, you can move onto another service if you need to (and even direct integration you can later abstract and move away from many of their services to alternatives). Same with Twilio; they provide SMS and other like services but others do as well so you can move your app to any platform with, mostly, minor code changes.
You can't make a minor code change to move your Facebook or Slack app to another vendor / platform. In fact that wouldn't really make sense.
My point is that we have certain implicit biases (I too hate FB and Salesforce as platforms) but those biases aren't really based on some kind of general principle that "proprietary platforms are always bad and dangerous".
Proprietary platforms run by untrustworthy companies are bad and dangerous.
But do proprietary platforms run by trustworthy companies exist? If so, what happens when a new CEO, product owner, etc takes over and makes changes that could ruin existing relationships? Seems inevitable for every platform to evolve to some degree; what if the evolution is simply not compatible with what you built? Does that make them untrustworthy?
It all seems kinda nebulous to me and I'm not sure I'd want to put all my cards in one basket :)
Where is this unicorn proprietary platform that is run by trustworthy company?
The gatekeeper function of the platform means they can shut you off at any time.
If instead, Slack published a protocol so anyone could author a client, then bots would just be normal chat users, for the most part, on par with regular users, and a bot would simply be something you deploy somewhere to sit in your channels, just like IRC bots.
I think Slack is a great app, very good UI, but I'm a little sick of proprietary messaging. We really don't need proprietary centralized messaging networks. Standardize a federated open protocol and let people compete on the clients and let the community agree on extensions to message payloads that clients pay attention to, or ignore.
ps. your https://actor.im/platform page 404s