Go walk up to a few professional chefs in restaurants and tell them that they are not passionate about the food they make because they have an external monetary obligation to cook it. And please tell us the results. Or even better: go do the same with nurses and doctors. They obviously turned their passion into a buisiness with monetary obligation. Bonus point if you talk to them just after a busy nightshift.Passion for something you like does not require external elements, you do it because you want to and when you want to and you invest only your own resources. From there you can ask for recognition, either by praise or by spontaneous donations, which adds up to motivation to continue giving free rein to your passion.
The obligation requires external elements, either labor or funding, that is, you ask for money or help from people to help you develop your passion, but that money and collaboration is not free. You can put all the passion you want into it, but from then on it is subject to dates, to results or to reminding you that every month you have to pay your assistants or that you have to meet certain requirements.
As you can see, there is a difference between "making a business out of your passion" and "turning your passion into a business".
You are arguing against yourself too, by the way. While the core game Stardew Valley was developed by one man as a passion project, the publishing was done on contract and it was sold for fixes prize, not spontaneous donations either. Even better, the developer wanted the game to have a co-op mode and that netcode was developed by the publisher. Was the multiplayer mode not part of his passion because he couldn't just do it by himself but had to ask someone else to pour their resources into it?
And currently he is slowly assembling a team to support the game further, so by your own words, he is not just investing his own resources into it anymore, so sorry, this is not a passion project according to you.
Passion is a difficult concept to discuss and it is also true that money and business decisions can and do erode the passion that might have existed in a project, but your idea limits the scope and scale of passion. Were the astronauts and engineers not passionate about reaching space or the moon just because nobody can do it alone? I think a space mission can be a project of passion to basically everyone involved even if the decision that allowed it to exist was economical or even political.
There probaby is a better definition for a passion project out there, but right now i think that if the people involved put their heart into it because they wanted something to happen or be created, then that is a passion project, regardless of most circumstances.