This makes the improper assumption that you're forced to upgrade. The code for Angular 2 will not evaporate when Angular 3 appears - at that point the choice to upgrade to the latest version is just that: a choice. You are free to wait until Angular 4 comes out before you adopt Angular 3 if you want to make sure the ecosystem around it is solidified.
I've been 'forced to upgrade' in the past by issues in dependencies that won't be fixed, with other dependencies not updated yet. It's a sticky wicket.
That said, breaking changes every six months seems to be commonplace in major open source packages. My initial reaction was 'OMG, now they want to repeat the Angular 2 debacle every 6 months?!?', which doesn't seem to be the case.
