Yeah, I can build a Slack "clone" in a couple of weeks with my own two hands, no AI required. But it's not going to actually be competitive with Slack.
Just to pick an incredibly, unbelievably basic enterprise feature, my two-week Slack clone is not going to properly support legal holds. This requires having a hard override for all deletion and expiration options anywhere in the product, that must work reliably, in order to avoid accidental destruction of evidence during litigation, which comes with potentially catastrophic penalties. If you don't get this right, you don't sell to large corporations.
And there are hundred other features like this. Engineering wants an easy-to-use API for Slack bots. Users want reaction GIFs. You need mobile apps. You need Single Sign-On. And so on. These are all table stakes.
It was a cliche for many years that Microsoft Word had "too many features." So people would start companies to sell "lightweight word processors" that only implemented "the most used 20% of features." And most of these companies sank without a trace (with a couple of admirable exceptions that hyperfocused on specific niches). Google finally made progress against the monopoly, but to it, they actually invested in a huge number of features.
Believe me, I wish that "simple, clean" reimplementations were actually directly competitive with major products. That version of our industry would be more fun. But anyone who thinks that an LLM can quickly reimplement Slack is an utter fool who has never seriously tried to sell software to actual customers.