If someone wants to learn a language just to enjoy its language & literature, and not necessarily be conversant in it, then would it be okay to never speak it? Or is conversation an essential practice?
Of course. For example, while there are a small number of "living latinists" who actually want to speak the language, the vast majority of people studying Latin just want to read it, and the teaching material reflects this (focusing on interpreting text rather than composing it or speaking it) Same with other classical languages such as Ancient Greek, Sanskrit, Ancient Egyptian, and Babylonian.
If you are asking, can I learn a language enough to enjoy literature and studying the language without engaging in conversation, the answer is "yes". I know lots of people who do this (especially in Japanese since a lot of people only study it in order to enjoy manga and anime in Japanese).
Interestingly, there is a prevailing theory that even if you never have a conversation, you can eventually acquire the language (see the work done by Stephen Krashen). There are examples of severely abused children who were able to acquire language without being allowed to converse. I would say it's not the fastest way to learn, but it seems it may be possible.
The main thing is that you need to be able to make a mental map of the language and then check your mental map against reality. Conversation is a good way to do that because it allows a native speaker to quickly point out problems. There are other ways to do it.
A lot of language is culture, though, so it can sometimes be hard to truly understand some language without having the same experience. Usually this requires experiencing social events with real people, which also usually requires considerable conversation. But if enjoyment is the goal, it isn't necessary to understand absolutely everything.
In that case you can never speak it and still achieve your goals. You'll find you can get stunningly far in your comprehension and totally clam up when confronted with an actual conversation that you will do dismally bad it. It's quite funny almost. They might as well be orthogonal skills.
It's very fun and rewarding to just dive into pop culture and literature and enjoy that one aspect. I've loved it every time I've done just that dimension. Though you inevitably wind up feeling like you want to speak with people.