How niche? What's the total market size? Whether or not you can make some income is one question... But can you make an income you'll be really happy with? Will

How niche? What's the total market size? Whether or not you can make some income is one question... But can you make an income you'll be really happy with? Will you be able to hire a replacement for yourself if you want to go do something different? What's the best case scenario?