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apologies for the overly sappy bits near the end--im just forever flat out awed by the geniuses from the early days of the interwebbers)
there are a lot of reasons why FOSS. a few of them:
1. Dont Repeat Yourself. i can't imagine how stunted we would be if companies, organizations, and personal projects didn't have access to simply grab a bit of code which had already been written and shared. the amount of time and energy wasted if they had to code what had already been solved would just be unfathomable. with sharing, we can just grab it and drop it into the project and move on to solve other issues. when i think about the society sized scale of where our code is used, open software and by extension, Dont Repeat Yourself has been just unimaginably important.
2. sharing knowledge is as old as humanity--its a large piece of how we thrived. if i share it, it now belongs to us, all of us. rich poor and everyone in between. all of us. there is no class involved, there is no gatekeeping. rich and poor alike can use it. i've seen a number of people either imply or outright declare that open source is anti-capitalist class warfare, which is just nonsense. its the opposite. if someone locks the knowledge away and limits who can use it, thats much closer to class warfare than choosing to share it with everyone.
3. i don't mean this next bit to sound pointed towards you at all, you seem like you're genuinely curious and likely read the talking point somewhere else. but i do wish people would stop perpetuating this myth: the stressful/financially unsustainable bit. if you don't want to do open source or your circumstances don't currently afford the time, etc.., don't do it. its that simple. no one is obligated to support or upkeep their projects. most of the active projects are kept up because the creators and community enjoy it. there are countless highly forked projects where teh author has clearly stated, "this is 100% as-is. i apologize but i just don't have the time or energy to upkeep or support beyond sharing the code." so they share it and thats..that. im not sure where this myth has come about implying that you're somehow automatically tied down for years or something. ive a few active projects which myself and others upkeep mainly because we enjoy it, enjoy the community, and the friends surrounding it. conversely, i also have many projects which i have set free and haven't touched since yet they still get forked. sure, there are cases where a few loud people will be annoyed... but i mean... oh well. almost always, if you're human about it from the beginning and make it clear "apologies, but i really wanted to share this, but i don't have the time to do anything beyond that" you're fine. people typically get it.
4. and a big one for me is i spent far too many nights, weeks, and months being omg_so_inspired reading old archived mailing lists, forums, books etc on the subject of the early days of the internet. these people werent discussing how they could lock away all of their work and hide it from the world. while there did seem to be some companies who were determined to build walled gardens, ultimately they failed miserably compared to the open internet. (beware the upcoming tangent: we currently see current iterations of them trying to wall us into their gardens with insta, twitter, tiktok, etc where this same group of investors are desperately trying to convince us "its only successful if everyone is in one place at once time". but we see these places crumble over and over again. we see this in so many graveyards of formerly semi-popular "social media" sites. yet the same group of investors still keep trying to convince us over and over again. "all in the same place! everyone here! if everyones not here its a _failure_." its pretty clear how it will ultimately end though... back to many spaces rather one--the open internet. we'll eventually realize humans are humans. every city has many many many different gathering places, not just one giant room but many types of bars, many types of restaurants, many types of parks, many types of neighborhoods, etc.) everytime this happens its just becomes more and more clear how wise the early internet thinkers were, absolute mental and heartfilled giants. they ran head first into big big big questions and kept coming back to "openness is the key for everyone"
i wasn't lucky enough to have been born yet to interact with these giants during those days, but i grew up seeing the outcomes. one thing that seems true time and time again, from infrastructure all the way to the end user, the open aspect is the key. its ultimately what keeps us on a somewhat level playing field, despite certain peoples repeated attempts to lock us into their personal fiefdoms. if we want freedom to leave and roam wherever we want, the only way to have that is openness. every time these same few investors try to convince you that "its only successful if we're all here, in my fifedom" we'll always have the freedom to roam to wherever we want.
many of us see FOSS as the only way to keep any semblance of true freedom because they will keep trying to corral us. whether they try to claim a 'town square [lol]' or whether they try to own our attention span, as the early internet giants figured out, one of the best foundations, one of the surefire ways we can keep from being corraled is sharing code.