There is nothing saying that the mia content will shut the game down at some deadline, therefore not "as vital" as hand job bit and can be added after.
You are mistaken. I will try to explain in a way which makes it clearer, though my previous explanation really should have been enough.
It doesn't matter whether or not there is a notice in the game that the Mia content (or lack thereof) will result in a game over. The Mia content has to happen earlier in the week than the handjob content. This fact alone makes it more "vital" for the progress of the game.
Consider what would happen if the handjob content, which has to be completed by Day 7, was added before the Mia content, which has to be completed by Day 5. Players who had already played past Day 5, completing the handjob quest on Day 6 or 7, would then have to start a new game in order to play through the Mia content. This problem doesn't exist in the current version of the game, because the content has been released in the order of how vital it is for story progression.
In other words, just because the content wasn't released in the order you would prefer doesn't mean that it was released in the wrong order.