The ARM ecosystem is way more open that the PC ecosystem. Think about it, instead of having a single (AMD barely counted till recently) CPU manufacturer designing and manufacturing the chips, you now have a company that publishes reference designs and several other companies build an array of interesting implementations.
The only reason why ARM seems more closed is that you primarily interact with ARM chips on highly restrictive platforms. The M1 chip will go along way to changing that perception as consumers will now be able to use an ARM based platform as they normally would a regular computer. But, that's not the only way, you can buy a wide array of ARM based devices with recent-generation ARM designs and very flexible IO.
The only reason why ARM seems more closed is that you primarily interact with ARM chips on highly restrictive platforms. The M1 chip will go along way to changing that perception as consumers will now be able to use an ARM based platform as they normally would a regular computer. But, that's not the only way, you can buy a wide array of ARM based devices with recent-generation ARM designs and very flexible IO.
e.g. https://www.pine64.org/pinebook-pro/