The individual nucleobases (cytosine [C], guanine [G], adenine [A] or thymine [T]) are made from the 4 most common elements, and appear to be modestly simple molecules. Is it so unbelievable that they naturally occur like methane or sodium glass?

One tree (us) either appeared first and dominated resources, or we wiped our the other trees. Evolutionary forces work at levels above species. Two trees can compete with each other.

If this happened it did so at the microbe level. Physical evidence would be extraordinarily difficult. If we did find evidence of another tree it would be so 'alien' to our tree that might mistakenly call it extraterrestrial.

And since panspermia is necessarily done with microbes, they would get outnumbered and munched by our microbes if they arrived after our tree dominated our planet. Unless other planets happened to give birth to hardier, more virulent microbes, then welcome Andromeda Strain.

I also don't subscribe to panspermia because first local origin must be reasonably proved unfeasible, but playing devil's advocate ¿what if our life tree outcompeted the others?

That's certainly possible, but not likely. Life began on Earth almost as soon as it was cool enough to do so, so we would expect to see life pop up over and over again. And it's highly unlikely that they would all get wiped out.