CI is not optional.
For language acquisition, CI is necessary, and no one disputes it. For full inclusion of all students, no one can deny that tapping into what every human is hard-wired for (i.e. language acquisition) is the more universal practice and responsible choice as educators.
CI is not a method.
The messages students listen to or read are received as Input. When students understand those messages, they receive Comprehensible Input.
CI is not speaking.
Teachers often provide CI in ways that involve a significant amount of student interaction (e.g. storytelling, questioning, acting). During this interaction, students could nod, for example, in order to express meaning. Students don’t have to speak, at least not before they’re ready to do so on their own (i.e. not forced). Speaking is Output, not Input. Some argue that Output is just as crucial as Input, citing Merrill Swain’s Output Hypothesis as support. Yet, in 1995, Swain—herself—called the Output Hypothesis “somewhat speculative” (p. 125).
CI is not explicit grammar.
Teachers usually explicitly draw attention to grammatical features in the native language, so there is no CI, let alone any Input during this instruction.
The only time explicitly teaching grammar might contain CI is if it is done in the target language. However, the messages don’t just have to be understandable on an equivalent basis (e.g. nōminātīvus = nominative/subject), but also in concept due to the abstract nature of grammar. Most students don’t even possess this knowledge of their native language, which is why conventional teachers lament having to teach about the native language grammar—in the native language itself—which also means no CI.
So, there is a low likelihood of providing CI while explicitly teaching grammar to the few who comprehend grammar. In that sense, CI and explicit grammar are usually incompatible for most students. N.B. “Usually…most…,” and not “even after 3 years…for all students…”
Beyond that low likelihood of providing CI while explicitly teaching grammar, there is also the role of purpose and utility. Few want to learn about language per se without any other purpose, and grammatical terms are limited in their application (vs. high-frequency vocab used in a variety of contexts).
There are reasons for teaching what is necessary (i.e. CI), and reasons for teaching what might* be beneficial (i.e. explicit grammar), but every teacher must decide when, and when not to provide CI.
*For whom? When?