Set ourselves forward 4 years later, and now El Salvador is regretting its decision.
"Regretting" its decision? You, ser, are LAUGHABLE. Is this topic supposed to stir the pot?

What about other countries adopting BTC as legal tender? Is there still a possibility? Or is it "game over"?
If you truly understood how Bitcoin works, or even considered it, then you would definitely understand that there is NO need for Bitcoin to be "Legal Tender" to be used as a currency.
That's the essence of Bitcoin. It doesn't need any approval, it doesn't need permission, from any governing entity to be used for transacting with one another.