That's an interesting take on FOSS - that's it's a free buggier alternative to "mainstream" software?

Linux is ubiquitous across many devices (you won't even know you're using it) and servers yet it's all based on FOSS. There isn't an alternative for many of those usage cases.

Browsers like Firefox are FOSS. The alternative is not less buggy, but it is less private and sells you to advertisers. But even propriety software like Chrome is based off an open FOSS codebase from Chromium.

Other software has no better alternatives. Look at VLC (for video), OBS (for streaming and video capture), Calibre (for eBook library management). There are arguably all the best in their class and they all FOSS, and that is just scratching the surface.

Tools like WINE are FOSS only but they are revolutionising gaming having been repurposed into the Steam decknfor example.

Eveb the software that might be characterised as "alternatives" to thebincumbant proprietary software servers a major purpose. GIMP (alternative to Photoshop) and Libre Office (alternative to MS office) are free but also now increasingly important do not require any online subscriptions and data sharing with big corporation. For many people that's hugely important - why pay money and subscriptions for things you can get for free at high quality?

FOSS is a huge ecosystem of software, all of it free to use, change and share.