A living wage is the target minimum salary for all workers, regardless of their position. The ideal minimum target salary for any position would be the market rate for that position or the living wage, whichever is higher.
Assume services are offered in the range of $5/mo. per user, or $60/year per user.
Sustainable annual revenue would then require a minimum of 1000 users ($60,000) per worker ($50,000), with the remaining amount covering infrastructure costs and other business expenses.
It is entirely possible to serve 1000 email customers on ~$250/mo. of server capacity.
I don't know where exactly different breakpoints would fall, but serving 2000 customers isn't that different from serving 1000. User growth allows for some combination of wage increases, hiring, and expansion of capacity.
The workload should not scale linearly. To some point, each additional worker can probably double the number of users that could be reliably supported, allowing wage increases, improved infrastructure, etc.
As of 2023 Proton mail had ~400 staff and ~70 million users. 3) A ratio of 1:175,000.
To be clear, I'm only thinking in the 1~5 worker scale at the moment :)
10 users is sustainable as a hobby. I'm already running my own mail server, now it just has a few more users on it, I've been here before and it's fine. I keep the servers patched and up to date, that's it. Once a year I answer a support question.
100 users, side project: Almost the same as 10 users, but now the server is paid for, and maybe some upgrades. Answer a few support questions a month.
1000 users is a part time job, 30-50 hours a month: Still just one server, but bigger. Answer a few support questions each week. Spend some time thinking about upgrades, backups, monitoring, etc.. Slowly add some new features or services.
2000 users is a full time job. Basically the same as above, just with less free time. Hire contractors to do web design, help out with this or that.
5000 users, 2-3 full time workers, lots of shared responsibilities.
10k users, ~5 workers, ~2 business admin, support, sales, etc., ~3 operations and development.
Minimum size of a BC coop is 3 members, which could be supported full time at somewhere between 3k-5k users.
After that, I think long term, robust sustainability probably kicks in around 10k users / ~5 workers. At that point, you can begin to absorb somebody suddenly leaving without it being a huge setback.