Judaism, Christianity and Islam are all religions that originated among Semitic people. Hatred of the practitioners of any can be considered antisemitism.

This is a very weird rule. Scientology originated among US Americans. Does that I can refer to prejudice against Scientologists as "anti-Americanism"? It seems to me like you believe that present-day Jews are not actually descended from the ancient Israelites, but you don't want to go so far as to say that antisemitism can't refer to hatred of Jews. So you chose a strange rule that would even count hatred of Japanese Catholics as antisemitism. If you think that Arabs are Semites and Jews aren't you, can just say that.

However you want to define it, it is not exclusive to the practitioners of Judaism.

It's indeed not exclusive to practitioners of Judaism, as some non-practicing Jews and gentiles mistaken for Jews also experience antisemitism.

The original definition that limited it to hatred of Jews is wrong and misleading.

It's wrong and misleading because the term itself comes from 19th-century racial pseudoscience mixed with biblical literalism. It persists only out of its inertia as the primary term used to mean "hatred of Jews". You can argue that the term should be retired for being inaccurate, but you can't argue for it to be redefined to something else that's just as inaccurate.

This would be like if someone made up the word "anti-Herculism" to mean hatred of Greeks, thinking that Greeks were descended from Hercules. Then after 100 years of people using the term that way, someone came and said that such usage was misleading and "anti-Herculism" should also include hatred of Italians. After all, Hercules was also a figure in the mythology of ancient Italy and because Italians and Greeks share a lot of cultural and genetic traits. Those things may be true, but they're beside the point. There is no correct technical meaning of "anti-Herculism" because Hercules wasn't real.

Judaism is passed maternally.

Jewish identity itself is passed matrilinially, but specific lineages are passed patrilineally: for example, status as a kohen or Levite. If "Semite" were a category in Jewish law, then it would be passed patrilineally too.

This is false. Arabs are hated for being Arab regardless of their religion or skin colour.

That link has exactly zero examples of hatred of Arabs on the basis of speaking a Semitic language. There are perhaps some people who hate Arabs for reasons unrelated to race, religion, or politics – but to say that someone's prejudice is against "Semites," defined by the language family, would require that they be prejudiced against speakers of Arabic, Hebrew, and Amharic, but not against speakers of Farsi or Turkish. That is not a real thing.

Ashkenazi Jews aren’t the only people in Europe with Levantine ancestry.

Your link said "Past research found that 50 percent to 80 percent of DNA from the Ashkenazi Y chromosome, which is used to trace the male lineage, originated in the Near East." What are some other European groups that are majority-Levantine in the male line?

If you are going to claim that some Levantine ancestry 2000-3000 years ago means you are a Jew,

I did not claim that. I was responding to your statement that "European Jews can claim it [the term antisemitic] despite having no Semitic ancestry." If you believe that there is such a thing as "Semitic" ancestry, then European Jews have it, with the exception of first-generation converts.

then there’s a whole lot more Jews than just those who practice Judaism.

It's true that many Jews don't practice Judaism. In my opinion, someone is a Jew if they identify as a Jew and if they either converted or have a Jewish parent. It is not just a matter of religion nor just of ancestry.

Including everyone in Palestine more or less.

Yes, Palestinians are descended from the ancient Israelites.

Because some European did something bad, Arabs have to suffer. This does repeat itself.

There are many examples of this, but being asked not redefine a word that has always referred to hatred of Jews is not one of them.