You have to do you own thing. Find something where you are good at. It doesn't work replicating someone else success because circumstances are different. And you might not have the same type of skills that are required. What worked 5 years ago is not relevant anymore. Especially on the internet things keep evolving fast.
That's why passive income and the internet are mostly bullshit. Nothing is really sustainable or passive. Even renting out properties is not because you have to maintain them and find new people who want to rent, do al the paperwork etc.
But sustainability of a business would mostly go hand in hand with it's competitive advantage. Why do people have to buy something from you and not from the competition?
Why are some corporations so successful?
Facebook, Ebay have a network effect.
Pfizer has patents to protect it's inventions from being ripped off by competitors.
Coca-cola has a very valuable brand name.
Some Chinese corporations have a production advantage because they can produce cheaper than Western countries.
Luxury brands like Gucci, Supreme, Versace they have very good marketing and can hype up the brand. In essence you just pay a premium in price for their brand name, to look cool. So they play on human psychology and use it to make a profit.
Amazon has a scale and distribution advantage over competitors.
You know what i mean? But it wouldn't work out starting your own Amazon, the sweet spot to enter that niche has already passed. You can't compete with them just like that.
It would be smarter to get into some new trends like AI or blockchain. There are still a lot of opportunities but you need to skills for it.
Create a niche for yourself , put original contents and be consistent. That way you will go viral, people will contact you when your service is needed and you will make money out of it.
Make sure you put yourself out there in a relevant way
Well I think there are lots of ways to make money online, but picking the most sustainable one is quite the task because there's no business venture that's perfectly sustainable, trends change and businesses become obsolete quickly these days, if not companies like Kodak wouldn't have gone out of business.
I'll list out a couple of business ventures that I think are reasonable though;
1. Freelancing: Sites like Upwork and fiverr offer wonderful services for you to properly sell yourself and make money from freelance jobs like writing, programming and proof reading, as long as your skills are still useful to the world then freelancing might just be for you.
2. Crypto-mining: Well there are alot of scam sites out there, but mining cryptos is still a pretty lucrative investment if you ask me. Cloud mining in particular isn't a bad idea at all and if you happen to plan things properly then you could end up making a reasonable profit.
3. Social Media Marketing: Well social media marketing isn't something that just anyone can do, you have to have a large following and alot of influence on popular social media sites, but if you happen to have that then making money from it would be a good idea.
I hope this helps.
If you're a writer, have a blog. I hear folks make a lot of money off it
There are lots of different sites that you can use to make a daily income:
@Grindabuck lets you click ads, do surveys, fill out forms and watch videos to earn giftcards, bitcoin and ethereum: http://grindabuck.com/register?r=lcnQ4dS509/HzNab
Earncrypto is the same way but lets you choose many types of cryptos to earn, including STEEM: https://www.earncrypto.com/earn-free-steem/?r=118972
Also, if you make STEEM posts daily, you will continue to earn an income online.
Find your nich, make it something you're passionate about (otherwise you can give up pretty easily), create content and be persistent,... I'm sure you could use certain permaculture principles to be "sustainable" such as "stacking functions" in the way you work your own marketing and all that!
There isn't one method. Think about what you are good at and work hard.