Conventional wisdom says you need the book for tenure at a research 1 university; that's the gold standard in certain humanities fields. (Two books for promotion to full professor.) Conventional wisdom also questions this standard, since publishers are cutting back on the number of monographs they put out.
If we move to a system de-emphasizing books, we will have to move toward an even greater reliance on peer-reviewed articles. This system is also problematic, because competition for the best journals might become even more intense. In the Humanities journals are not ranked as clearly, with clear consensus about #1, #2, #3. In some fields there are clear expectation that you need to publish a certain number of articles in the top 2 or 3 journals to get tenure. If we remove the monograph requirement, will we move to a model like this? Then any imperfections in the peer-review system would be exaggerated, because all it would take is for one of the three to be poorly managed to throw a wrench in the works.
My colleagues coming up for tenure have still been able to publish books. I've still been able to publish books. We could argue about just how difficult it is to publish a book before we would have to abandon the 1-book rule, but we are not necessarily there right now.
A book is not just an article x 5 or x 6. It is qualitatively different, requiring a larger conception of things and a more sustained effort. Some say articles can be just as influential, but then a book containing several previously published articles is even better, right? Books receive reviews; articles do not.
The argument that people who only write brilliant articles, and never books, write more brilliantly than those who write both, finds support in the work of a very few brilliant scholars, like the late John Kronik. It is hard to set that up as a model, though, because that is the exceptional case and not the norm. The most influential humanities scholars write books and will continue to do so for the foreseeable future. If we say you can get tenure without a book, we are saying you aren't going to be all that influential.