Theory: perhaps the letters do not have phonemic content?

[ololololo]

Here is a recent example: You are not allowed to view links. Register or Login to view. that contains 400 PAGES of enciphered dictionary articles (for verbs only) from a know dictionary. Would you call this practical?

Very interesting example! But I don't consider this proof that Voynichese is a artificial language. It's impractical, but by the looks of it, it was worth it.

As a counteroffer I suggest you try to create your own mysterious manuscript that would grab everybody's attention and then would resists all attempts of making sense of it for hundreds of years

With modern technology, it's not that difficult. Create a random set of letters, add a system to it, add some similarities with other systems, and (most importantly) Tell me you don't know where you got this book from.
If we talk about meaningful text, then we can simply complicate numeric substitution in non-mathematical ways. Theoretically, it will be possible to decipher it, but without the alphabet and any information it is very difficult...

If its topic turns out to be about summoning demons, divine revelations or plant magic rituals, the impracticality could be an essential part of the process

It is your responsibility to prove why I should believe that you are right and that the VMS contains some kind of mystical subtext  I've seen many such theories, but they all don't seem convincing enough to me. For example: You are not allowed to view links. Register or Login to view.
I think I've proved why the philosophical language version is unlikely. By the way, your counterarguments do not prove this theory in any way. If you say, "actually, other people did that at that time", it doesn't mean anything by itself, they did a lot of things in the Middle Ages.