I look at it like this: the percentage of most people directly using FOSS vs proprietary software is 0%.
If I can get them to use even one piece of FOSS software, that increases the percentage, which is a win for FOSS.
I moved my parents onto Linux Mint a few months ago. They still use Spotify, Gmail, Chrome, Outlook, Onedrive, etc. But they are doing that through Linux, and I got them to switch their office suite from MS Office to OnlyOffice.
So instead of them being 100% on proprietary software, they are now using Linux, which protects them from malware and Microsofts spyware and bs, and makes them aware slightly of FOSS and how good it can be.
That is a net win as far as I can see. FOSS usage just increased slightly, and proprietary decreased, no other tradeoffs were made.