This. I'm an iOS developer and only working and learning new things in the field consumes all my time. I can't image how would I start working on different technology (say node.js, which is something I want to) without impacting on my iOS learning curve.

Ignoring the more generic point the gp poster made, if you want to learn Node but not sacrifice expertise on iOS why not identify a mobile app that requires a server side component and do that in Node. That way you still feel like you are working towards a singular goal of doing app development.

Not really, no.

Building a high-quality app, from scratch is indeed time consuming and generally a full-time affair for a single person. However maintaining it afterwards is significantly less effort as is replicating it to another platform. Latter is not a trivial effort by any means, but it is still much less time consuming than writing an original, because you can readily recycle (a) code (b) ideas (c) supporting infrastructure such as website, support pipeline, etc.

I agree. SongSheet on iOS is consuming all my time right now, but 6 months from now I'm sure I'll be spending time doing other things (porting SongSheet to other platforms or even developing other applications).