The scalability problem with FOSS is monetary and motivation.

The successful products need longterm financial security in order to plan and support their peoduct(s) - so, do we start seeing more subscriptions as corp. sponsorship fades away?

And, just like XKCD 2347, FOSS needs to step up and support the components they rely on

That's going to need some more maturity from the developers too: it's a great feeling doing something new and interesting, but - like having a pet - you can't just abandon something when you're bored of it, or too busy, without rehoming your project(s)...

That's where I see the industry needs to improve before they're really ready for the big time.

From enduser perspective the most visible change would be that all software wouldn't be hostile to users because with propreitary you have to be very picky to get that.

In the long term we would see that companies could not build walled gardens to block off competition. Contrast Windows & MacOS vs Linux with its different distros, DEs, toolkits etc.

The least difference would be for enterprise because support is expensive either way.

A significant decrease in the amount of surplus value society produces going towards tech companies producing proprietary software, whicj is most of them. Basically the costs of using software for a whole lotta things are gonna get lower. This would make that society's products cheaper for itself and export. It would allow its labour to do more useful things, one of which could be new FOSS software. But also helping out with the green transition, taking care of the ageing population, education, etc.

Politicians will likely take a more active role in screwing it up.

Realistically, probably not much for people outside of the tech industry. People will use the best tool for the job, and whether it's foss or not won't matter.

more people contributing to the code

One huge impact mass FOSS adoption would have is that there would be a lot less software and hardware churn. Commercial nature of proprietary technology is the main driver for constant upgrade cycles we see. Companies need to constantly sell products to stay in business, and this means you have to deprecate old software and hardware in order to sell new versions of the product.

Windows 11 roll out is a perfect example. Vast majority of Windows 10 users are perfectly happy with the way their computer works currently, they're not demanding any new features, they just want their computer to continue to work the way it does currently. However, Microsoft is ending support for Windows 10 and now they're forced to buy a new computer to keep doing what they've been doing.

This problem goes away entirely with open source because there is no commercial incentive at play. If a piece of software works, and there is a community of users using it, then it can keep working the way it does indefinitely. Furthermore, in cases where a software project goes in a directions some users don't like, such as the case with Gnome, then software can be forked by users who want to go in a different direction or preserve original functionality. This is how Cinnamon and Mate projects came about.

Another aspect of the open source dynamic is that there's an incentive to optimize software. So, you can get continuous performance improvements without having to constantly upgrade your hardware. For most commercial software, there's little incentive to do that since that costs company money. It's easier to just expect users to upgrade their hardware if they want better performance.

I would argue that non technical software users would be far better off if they had the option to fund open source software instead of buying commercial versions. Even having to pay equal amounts, the availability of the source puts more power in the hands of the users. For example, building on the example of Gnome, users of an existing software project could also pull funds together to pay developers to add features to the software or change functionality in a particular way.

This is precisely what makes licenses like GPL so valuable in my opinion. It's a license that ensure the source stays open, and in this way inherently gives more power to the users.