'After a semicolon, you need a complete sentence' ???

Well in your mind, what constitutes a 'complete' sentence? We're taught that it must contain a subject and a predicate, but must it?
I'm not trying to be a smart-ass here, but grammar 'rules' change due to English usage. If suddenly 'I very like' became common usage, then it's perfectly OK to use it, despite the rules saying otherwise.
Since all 'sentences' need a verb, is 'Go!' or 'Stop!' a 'complete' sentence? - Many people would say 'No', but others would say 'Yes'.
If you insist on a predicate then what about 'I am' as a contender for the shortest sentence?

As for your semicolon argument - how many sisters do I have in the following sentences? Or rather, which sentence is more clear as to the number of sisters I have?
Do I POSSIBLY have 8 or DEFINITELY have 4?

I have a sister in Nanning, Guanxi, also Shenzhen, Guangdong, as well as Nanjing, Jiangsu, and Chengdu, Sichuan.

I have a sister in Nanning, Guanxi; also Shenzhen, Guangdong; as well as Nanjing, Jiangsu; and Chengdu, Sichuan.

Both sentences are 'grammatically correct', and as it's ME that's writing the sentence, I can choose which one I want to write.

Personally, I think that FT's have enough 'rubbish' to contend with, with Chinese-English teachers telling kids they are 'wrong' when they are not - (Oh, and did I just use the word 'with', TWICE? - That's grounds for 'execution', right?)
You may have seen from one of my previous postings that I complained about a Chinese teacher telling a student he was 'wrong' for saying 'We will have a party next Saturday', instead of 'We are going to have a party next Saturday'. The reason....?; well, in Chinese they would say 'We PLAN to have a party next Saturday' and as such, 'We are GOING to....' is a much 'better' choice - at least in HER opinion!

Then there's the colloquial uses and the differences between American English, British, Canadian, Australian etc etc etc.

For me, if kids are 100% wrong, as in 'I very like', then I give them slow torture, but if they are not 'wrong', in a 100 years who will care anyway?

Just as a footnote, consider this for a moment......

If you had a time-machine that could travel 500 years back in time or 500 years into the future, and you used it, how much English do you think you would understand and how many people do you think would understand you?