I'm wondering what's appealing about this size board? I'm interested in portable devices so I prefer smaller. Arduino's boards seem awkwardly big and expensive compared to a Raspberry Pi Pico, Adafruit's Feather boards, or a Teensy for more processing power.
The UNO predates the Pi Pico by about 20 years. It was pretty small for everything it did back then. The headers are laid out in a standard format so that it can accept "shields", so now anything that wants to use those shields has to be this size.
The first Arduinos with that formfactor which supports shields are from around 2005[0]. The Uno wasn't the first[1], it was just a model line set up for that specific formfactor for consistency, since the name was changing frequently (e.g. "Arduino Diecimila", "Arduino Duemilanove", etc).
If you're not interested in shields, I don't think there's any reason to use any type of Arduino today. There are smaller Arduino boards that don't support the same shields for people that want something smaller. But, today, the ESP* boards are way, way better. And, as you mentioned, some of them have shields too (though not anywhere near as many as the UNO).
I like the feather format a lot as well and Arduino makes some in that shape (and smaller) but having taught a number of beginner's classes with Arduinos at my hackerspace, I think the Uno format is a bit friendlier for novices. Having a wifi/bluetooth enabled board in that shape is great.
It's easier to see what you're doing and plug a rats next of wires into the bigger board without it falling over, and everyone's got to start somewhere! Uno format Arduinos are a learning tool first and foremost and they do that job really well.
According to [1], the decision to keep the spacing for compatibility reasons was a conscious one. I can see how they might want to keep the whole ecosystem of Arduino-compatible accessories/kits/etc functioning.
Being physically bigger makes it easier to work with, as there's just more room for everything. The smaller Pro Micro/Pi Pico/Feather/Teensy form factor is just a bit too cramped for prototyping.
Backwards compatibility. Plus you can power it with anything between 6 and 24V, use a coaxial connector and the header is big enough to attach just about any prototyping board or extension. We yse the same header on our own ESP32 extension board, but replaced the coax connector with something more industrial friendly.
Now it also runs Micropython, so we don't have to bother compiling code, you just push it over UART and run it.
Another big shift is going to happen when these boards shall be able to run Node Red. Siemens PLCs now run Node Red. I don't like it, but many engineering professionals have a better experience using block programmingand get stuff done even if it's a waste of resources.
They have the maker devices if you want a smaller board, but it's rather expensive compared to the alternatives.
Feather seems like a great choice for boards of this class.
From the Arduino Uno R4 FAQ…
> “the Arduino UNO R4 WiFi was specifically designed to ensure compatibility with previous shields and compatible hardware developed for the Arduino UNO R3. The UNO R4 WiFi maintains the same mechanical and electrical compatibility, allowing you to seamlessly use your existing shields and hardware with the new board. This makes it easy to upgrade to the UNO R4 WiFi without the need for significant changes or adaptations to your projects.”
In addition to physical compatibility, maintaining the GPIO pin layout means software libraries created for earlier Arduinos should continue to work. For example, an interface library for a sensor or actuator that’s reused project to project.
They keep making them, so there must some reason?