Hi Federico,
thanks for your question.
Our chosen approach for the new mobile font stack is explicitly applying system fonts first, then established web fonts (Helvetica, Arial), then a generic font (sans-serif).
The iOS 9+/macOS system fonts have to be included otherwise, browsers would fall back to Helvetica, which it does on macOS < 10.10.
Roboto is intendedly included for the wide-ranging variety of Android based systems and browsers, ensuring Roboto where available even when specific browser default should be different.
Helvetica and Arial are included in contrast to the former Typography Refresh as we haven't found any issues in our testing with Linux operating systems (Gnome, KDE, Ubuntu Unity) and it provides a calculable fallback for older, more rare use cases (MinervaNeue on desktop is still the exception).
Also given the prior art examples we decided to stay with a more explicit stack definition. Additionally, removing Roboto would have saved 6 bytes, removing Helvetica, Arial another 17 bytes after gzipping. Savings didn't seem useful when ensuring font choices that are aimed to improve reading experience for a wide-ranging audience.
Please let us know if you have further feedback – preferably on the task[0] or the project page in order to collect for future insights and transparency for people outside of the Foundation orienting on our approaches.
All the best,
Volker
--
Senior UX Engineer, Lead User-Interface Standardization
Wikimedia Foundation
volker.e@wikimedia.org | @Volker_E