I think there's a subset of tech folks that don't really value UX (which is like 90% of hey's value-add). It's the same people that don't understand Slack's value because "it's just over-hyped IRC"
A surprisingly large subset of those tech folks can and does improve the UX of their Mail and Chat life using basic scripting on top of a simple but flexible UI.
Both Slack and Hey say F U to that and force you to do everything your way. No standards compliance, no tailoring of the UX to your actual needs.
Focusing on hey for a sec: I understand that some people might prefer a system where they can take various components and combine them how they like best, in the same way some people like to cook their own meals.
But what's weird is this sense of anger at products that aren't as flexible. It's like being upset that a new restaurant opened up because they have a set menu and don't let you make your own food. If you like cooking your own meals, great! Keep doing that. This restaurant is for people that want someone else to make a meal for them.
With slack I understand the frustration a little more, since it's something an entire company is going to be using and asking you to use.
Counter point: Hey is asking you to commit for a year, if I was going to the same restaurant every day for a year, assuming the other ingredients are on the menu elsewhere, I would expect being able to make substitutions and change meals up a bit.
I think the metaphor starts to break down a bit here because variety is usually specifically desirable when it comes to food, but not really when it comes to email or any app's workflow.
I don't see how that meaningfully changes the metaphor? Imagine, idk, someone who likes to very carefully design their apartment vs. someone who hires an interior designer to do it for them. People that love to tinker with their cars vs. people that don't.
Some people like to very carefully craft an experience on their own and some people are happy to have someone else do that.
As far as the software ecosystem is concerned, it is compliant with SMTP, obviously. But if you are going to use an IMAP client, you could as well use any of the zillion email providers that support it, or set up your own mail server.
It's like operating systems. People who want to have it their very specific way use Linux. People who want it to just work out of the box use macOS (but not Catalina).