I am thinking about open-sourcing one of my projects but I am not sure how to sustainably keep working on the project if the customers get it for free.
How exactly does Joplin and its core maintainers get money for their work?
I didn't see any paid features or hosted versions of Joplin.
https://joplinapp.org/donate/ is the more obvious answer.
The rest is that people often do make passion projects for free and help out without the reward of monetary gain. I'm not putting words in anybody's mouth, people might be doing development for other reasons but fully open community stuff isn't exactly rare in the FOSS world.
I understand that some people love working on FOSS but even if I do myself like that, I still have to earn a living somehow. I can't afford to spend 8 hours/day working on a project for free as I wouldn't be able to pay my rent.
Donations sound like the obvious solution, but I am not sure if those are enough to allow the sustainable development of the project. I know for example that Vue.js for had a pretty successful Patreon campaign, but it seems like an outlier. Are donations enough?
I have an open source project, not related to Joplin. The first year I got some donations, but once everyone who felt like donating had done so, it trickled down to basically nothing after the 3rd year. Bottom line: expect very little money. Definitely don't expect to give up your dayjob over your project!
Edited to add: Joplin is still growing, so I would imagine that donation are still flowing freely. I have no way of knowing that for sure, I'm just guessing. But at one point that will be saturated as well. Take products like Plex/Emby, Drupal etc. They all turn commercial in various degrees after being open source for a while.
Not saying that will happen with Joplin of course, I'm just saying that depending on donations is probably not the safest course of action.
In my case I already quit my job and I am already selling the product (for money), but I would love to somehow be able to give it for free while remaining sustainable. I think there's also the possibility of having some high-value clients/donors that would cover the development costs and distribution for everyone else, but this again sounds pretty risky.
I also prefer for the product to be intensively developed instead of hoping that some external maintainers have the time to improve it.
I was asking maybe Joplin has a nice success story of how the sustainability problem was solved.
People do things in their free time as well you know, outside of the normal working hours and not like there is much else to do in the virus ridden lockdown hellscape that is the outside world at the moment.
I'm no programmer but I'm learning in my spare time for fun and hope to be able to contribute to projects at some point in time. My normal job isn't related to anything like this at all.
Of course the other beauty is if your project gets off the ground then you have a whole community of people donating small amounts of time and effort to it which can really add up. I don't see it as any different from people who have youtube channels with no monitisation or people who used to make flash games, community forum moderators, wikipedia editors etc. etc.
Does your product have anything in it that can be monitised as such beyond the initial cost?
I don't think there's any perfect solution to be honest, or else everybody would quit their job and have fun developing their own open source project 
Like Sophia, I'd say don't do it for money because even if you are very successful the revenue still won't compare to what you could earn as a full time developer at a company. For me part of the "payment" in open source is the enjoyment of working on something I like.
So it's best to do it for fun at first, without quitting your day job, and see where it's going. Maybe it could lead to a small side business after some time.
It's self-hosted and I prefer to keep the entire product as a whole and not go the open-core way (where you only provide some of the features for free).
I'd say don't do it for money because even if you are very successful the revenue still won't compare to what you could earn as a full time developer at a company
Maybe it could lead to a small side business after some time.
I think in my case I am considering the other way around. I am already at the business part, but this means only people who can afford can make use of my product. I am thinking if there's any universe where you can run an open-source project as sustainably as a business, but from most comments and other projects I see that it's:
A) Either the open-source project grows up to a point and the core maintainers have a hard time managing the product
B) The open-source project gets acquired or is part of a larger organization
C) The open-source project lives happily, only that progress is really slow and quality is nowhere near a premium product
We don’t know what you’re selling but to be honest if the business is not profitable I doubt going open source is going to save it.
You’d be best investing your time in market research, marketing, etc. Perhaps open source would give your business some visibility but I’d be surprised if it makes a difference in terms of revenue.