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.