Some notes about your requirements:
1. What you mean by "wellbeing"? A company - outside of government investment, like defense company - can only exist if people pay for their product. If they voluntarily pay for it, doesn't it mean they want it? Or are you allowed to argue that they may want it, but you think they shouldn't so it doesn't qualify as wellbeing? If so, you basically define what "wellbeing" is and I can't hope to guess what it is.
2. Again, what is "net benefit" - how you count it? I know one way, see above. You probably know about it too, and yet added two separate special requirements - so you probably have something else in mind. Again, it would be hard for me to see what.
3. Why not? I didn't have an urge to learn Spanish until I learned about Duolingo and an urge to learn about variety of subjects until I read Coursera catalog. It just didn't come to my attention it's possible for me to do it that easily. Now I can. We can go further - before air travel existed, I'd probably never thought I want to visit another continent. Now I know I do. Does it mean air travel didn't change the world? Of course it did.
4. What is "adequately"? Obviously, if people are paying money for the new provider, they find something in it that wasn't covered by the old provider. Again, you can claim people "don't need it" - like, they don't need smartphones, just sending ravens to each other and occasionally having a maester write a long letter is perfectly fine, but who decides that? You do. And I can't hope to match your criteria here. Of course, people lived lives in 600 BC, and by all indications were not hellishly unhappy, at least most of them, so should we claim all that happened since was unnecessary and didn't really change the world? Makes no sense to me.
> examples of SV startups that have, in your opinion, changed the world?
OK, here are some examples that made the world be different for me. Note I don't consider how good the change was and how good are the companies at day-to-day operations, only whether they did something that changed something in a big way. And I only consider relatively new companies, e.g. IBM won't qualify even if they invent time travel. Also, I do not discuss Google, because a) obvious, b) not a startup for a while.
Twitter. I mean, US president is using it and everybody in the world is jumping around it. You may not like it, but you have to admit this is happening. It's a different world now.
Square. The enablement of small businesses driven by it is phenomenal. And it certainly changes my patterns of behavior - I no longer need to run around looking for ATM (or, in many cases, just not patronize that particular business because it's too much trouble).
Yelp. Completely changed how people choose where to eat and who to hire and so on.
Uber. Changed the world for me - now in any city on the planet that has Uber I can be sure I can get whereever I want. Bus broke down? Train has 2 hour delay? Car in the shop and I need to go somewhere right now? I know what to do. Yes, taxis existed before - and my experience with them sucked (once I was stranded in the middle of pretty large US city and was told by dispatcher no cabs are available in the next hour or so. I had to call the only person I knew in the whole city - which was barely an acquaintance - and beg him to take me to the hotel. Very unpleasant).
Facebook. It is criticized a lot, and deservedly. But I now am talking to people that I literally haven't seen for decades. Yes, it's not "real" talking like sitting in the same room etc. This would probably never happen - we are too far from each other, and too busy, and too... well, a lot of things, but at least we're not 100% disconnected as it would be before.
Automattic. This is the company behind WordPress, which made publishing accessible to common people. Now everybody can do what in the past only major newspapers and writers could do.
Coursera (also Udemy) - not unique, but hugely game-changing thing, you can now get an several degrees worth of education completely free, not leaving your home and needing nothing but an internet connection. Of course, yes, getting a formal degree in a college has its benefits, but now it's not the only way.
BitTorrent (along with predecessors, copycats, etc.). This can be arguable whether it's good or bad, but the way that content is consumed online has been changed forever when peer-sharing networks appeared.
Netflix, as a complement to the above. Changed how people consume entertainment, and more. I mean, netflix and chill, right?
Mozilla - without it existing, Internet probably would be owned by Microsoft now, and Chrome would have much chance to happen either because everybody would think there's no chance to beating Microsoft there.
Apple, of course, may not qualify as a startup, but deserves a honorable mention by making the concept of smartphone finally work. One can argue that'd happen even without them, but somebody would have done it, and mobile computing has changed the world, and Apple took a huge part in it.
Wikipedia (not your regular "startup", but can be seen as part of SV) - need I explain anything here?
Of course, the geographic criterion here - only SV ones - is kinda limiting and arbitrary, but I guess it's a decent sample anyway. And probably missed a lot of viable candidates, it's just a list that I could make without starting a doctoral thesis on it.