As a developer 70% of my time is in XCode and Github. Guess where the rest is? Google Calendar and Google Hangout. I discount Gmail because I just use my phone for that, swiping away messages while compiling. Want to know where my boss is from 7am to 7pm every day? Google Calendar and Google Hangout.
Calendar is a really bad product that needs disruption as well. I imported my victor ops on call schedule the other day and it imported wrong, so then I had to go through an entire year of two meetings a day fixing it, something that would have taken 10 seconds if they let you search and delete results.
Re Mac only, the first stage of a startup is finding product market fit. It isn't about making your product work on every platform. In fact that's counter productive. The goal is to have the shortest build-release-learn cycle, if you are porting to multiple platforms, you just increased your build time, which increases your cycle time, which means you will take longer to reach product market fit. Once you do, you can go port to every platform, but it's harmful to do it early.