First of all, we are not necessarily talking about 万葉仮名 as a "complete writing system". Remember that Yudan's sentence could also be interpreted as "the specific kanji in iroha using 万葉仮名" in which case neither the kanji nor the usage is archaic.furrykef wrote:I think the point we're getting tripped up on is, if a modern name is written in what was historically 万葉仮名, it's hard to argue that such a particular name would be equivalent to writing it in runic letters, and I don't think anybody's arguing that it is. But 万葉仮名 as a complete writing system is a good analogue to runic letters, because it is not in modern use for that purpose and a document written in it will generally not be understood except by people who specialize in it.
And also remember that 万葉仮名 was never intended to be a "complete writing system" - it was devised originally for the specific purpose of capturing proper names, and is still used in that sense today. So, as a writing system, it is still being used today in the same way that it was originally designed to be used.
Now, we know that there exists literature written using 万葉仮名, iroha being an example. But it was never widely used this way.
So the point remains: we are not talking about a writing system that was used widely in the past, but is no longer used. It was invented for a specific purpose, and continues to be used for that purpose. There was a wider application of it to capture spoken language, which is no longer used (except possibly in special circumstances).
The real point here is that Japanese writing systems are not mutually exclusive. One writing system did not "replace" another. It's not as if 漢文 was replaced by 万葉仮名 which was then replaced by ひらがな and カタカナ. All these systems more or less operated side by side right up to World War II. The script reform that happened post World War II did simplify things, but all systems are still in operation today. 漢文 for 漢語, 万葉仮名 for (some) proper names, ひらがな for native Japanese constructions, and カタカナfor non-Chinese loanwords.
There are people who would like to see 漢文 and 万葉仮名 become obsolete. Specifically by replacing them with カタカナ. This would greatly reduce the amount on 漢字 used, and arguably will make it far easier to master the writing system. One of the aims of 当用漢字 (and to a lesser extent 常用漢字) was to encourage this to happen.