Most people I know don’t even know WhatsApp supports Markdown.
That said, none of them are complaining because it’s a chat program. You don’t need heavy formatting in a chat program, it’s meant for short and fast messages, unlike email.
As such, I don’t believe people will spend a lot of time formatting their input in Slack, unless they really are thinking people will replace their email with Slack.
Excel also happens to be the software with the most arcane UI on Earth.
'Making sense' is not the issue here, motivation is.
And I don't think removing a power-user feature and replacing it with something half-baked will motivate your user base.
P.S. WhatsApp uses a markdown-like UI for formatting: https://faq.whatsapp.com/en/android/26000002/
Somehow, its non-techie users aren't complaining.