To take the OP's points one-by-one:
- Hackathons are worth the commitment. They're a fast, efficient way to try out 1) new frameworks and APIs, 2) new employees/cofounders, 3) new ideas.
- Hackathons only exclude people with "lives" who don't choose to make the event a priority. Hackathons filter out timid and less motivated people (and those two attributes are related) and you're generally left with pragmatic people who will make it through the weekend.
- The OP is right. The inconvenience isn't evenly distributed. Life isn't fair.
- Hackathons are only as unhealthy as you make them. Gardening can also be quite unhealthy if you don't wear sunscreen.
- Competition is a positive force, and the extrinsic motivation of judgement and a deadline are real, and good for the hack you produce.
- Since this is a competition, you can't work on a preexisting product any more than you could start a marathon a kilometers up the road.
- They're not just toys. Take a look at POWr.io and Zaarly, both of which came out of hackathons. There are many others, and if anyone knows of some of the top of their head, please reply with them.
I think it's great that people are coming together to help code on your gardening project. Hackathons may not be right for you, but they are definitely right for me and the dozens of ambitious people I've coded with at hackathons over the years.