IMO Mbed was just as easy for hobbyists but had a far better designed API that could support professional work as well. Arduino is just badly designed.

Unfortunately the Mbed guys stuck to their crap web-based IDE for waaaay too long, and when they finally realised it couldn't cut it, they pivoted to Yotto, which was a terrible Python based build too. When that failed they finally made Mbed Studio which was based on Theia (same as Arduino is now) but by then it was too late.

I think they also lacked an obvious "start with this" board like the Arduino Duo.

I think if they have blessed one of the Neutrino boards (which were incredibly cheap and powerful compared to Arduino) with their branding, and switched to Theia like 5 years earlier they might have had a chance.

Real shame because it really was a far superior software system.