Software developers, how viable is making your own app?

[OverThinkMuch]

I'm a web developer and I've daydreamed on the idea of building a web app in my spare time.
...
- While I could probably find time for dev and upkeep, maybe support too... marketing is a job in itself

Focusing on that marketing for a moment, are you sure that's the only missing skill involved in the app? For example, you mentioned being a web developer, which is front end work. Do you have backend experience?