Windows 10. It was during the pandemic (late 2020), and I saw a Mutahar video of his desktop (at the time, I did not know of KDE Plasma, just gnome, unity and cinnamon) and I was like “Whoa, his desktop looks so much better than when I remember using linux. I should install Arch because that is what he used to get that desktop.”
I have used linux before on Fedora, Mint and Ubuntu, so installing arch using a youtube tutorial was not going to be that hard. Although it did take 2 days (Mostly procrastination and fear).
I will say this: I have a 98 computer and an XP computer for me to use, and I found those UIs better than in Windows 10. When I switched to linux with KDE Plasma, the oldschool UIs could not compete. Plasma is just THAT good.
I was also madly in love, with me calling KDE Plasma like being in a dream, and using Windows 10 is like waking up to the cold old stale office life.
What great timing too, with Proton kicking off right at the same time too, eventually me removing the need to dual boot.
TL;DR: I switched because I found out about KDE Plasma, and linux gaming was becoming infinitly better.