There are certainly use cases for which it's more than is required. Like the most simple would be adding a cron string to a GitHub Action or Vercel function, but in most cases, and certainly Slack's case, you want more reliability, scalability, flexibility, and/or observability. And Temporal has extreme levels of those things, including pausing & editing schedules and seeing the current status of all triggered scripts/functions and all the steps a function has taken so far, and guaranteeing the triggered function completes, including ensuring that if the process or container dies, the function continues running on another one. Even if you don't care about all those things, you might care about some of them in the future, and it doesn't hurt to run a system that has capabilities you don't use.
Depends on how many cron jobs you have and what you need out of it?
Operating Temporal is not that hard -- you can start with `temporal --dev` on your own box. I have a "Nomad-Temporal" Terraform Module to stand one up on Nomad. [1] Temporal has Helm Charts for Kubernetes [2]. There is also Temporal Cloud [3].
That said, there is currently a chasm between "script in cronjob" to "scheduled task in Temporal". The focus of Temporal is more "Enterprise, get your Business Processes on Temporal", not "soloist, ditch your cron".
There's certainly space for somebody to a make DAG dataflow thing or lower-code product over Temporal. Airplane.dev [4] was built on Temporal and was approaching this; acquired by AirTable.