You've asked a lot of good questions.
1. Cycling is more expensive than running. Period. Running is the cheapest sport on earth. I don't really agree that cycling is "gear-focused". Some people really like geeking out about the gear, but if you get a good bike you don't have to worry too much more about it than that. Unless you want to. I know a pro who showed up to workouts in shitty weather on an $800 bike (he didn't want to damage his good one), and I've been totally happy riding and racing with my $3k road bike. I have a Specialized Tarmac and absolutely love it. Bottom line, you won't need to worry about gear if you get a decent bike and learn how to change a flat. The bike shop can do everything else for you if you want.
2. The damn bike breaks down sometimes and it's annoying as hell. People leave sharp things in the road and you get flats, you need to replace the chain sometimes, and other stuff breaks occasionally. Better the bike than your body I guess.
3. It's also a different sport. Road racing has tactics. The tactics are frustrating at first, but when you start to figure them out you realize they can make bike racing more fun than running.
4. To be a competitive cyclist you have to spend more time on the bike than you did running. That's one downside of the lower impact -- everyone else spends more time on the bike, so to get to the same relative level you will too. It's like asking if you can be a world class 10K runner off of 25mpw. The answer is no, not really. The upside is you can do beautiful rides in beautiful places -- I spent four hours yesterday riding through redwoods.
5. Compared with road racing, mountain biking can be more like running in many ways (in some types of races like XC you just go hard the whole way), but it's also a lot more technical than road racing and takes longer to learn to be good at (IMO). You need to learn to go over obstacles without crashing and if you really want to get good at it you basically need to spend time practicing difficult sections of your rides. There's also a higher likelihood of crashing but the crashes are sometimes less catastrophic than road riding. Gear matters about the same, but you won't have to deal with other people as much (no group rides and tactics like road cycling).
Cycling is a new sport, if you want to be competitive you'll have to bite the bullet and learn it.
There's more I wanted to say but I'm getting tired. Night.