I am publishing a book in passages here myself and I don't see that as a big problem. Two things need to be considered:

1. You don't publish in chapters but in small passages (400-600 words), so a reader who finds it after weeks or longer there is a good chance there are fresh passages younger than one week.
2. As you wrote yourself, you have to publish continuously, so when the first book is finished, you publish the next one. So, over time you can collect a readership.

What I would find more important is the possibility to resteem old content. Currently, that is limited to 7 days as well and the consequence is that old but good content can't be rediscovered.

I will follow you now.