Crawler is possible, still need to get into the line though, I recall there being a few options for tethered camera crawlers meant for sewer inspection. Visual does have drawbacks, can't really size defects, as far as I recall it's difficult to get full coverage and cleanliness is even more important, and you'd general need the operations on that pipe to cease. Ideally you want your inspection regiment to allow you to know something's coming and be able to plan for it, example if I start seeing vibration increasing on some bearings, I can monitor them and start planning for their replacement on a scheduled shutdown.

No inspection is actually a totally valid mitigation plan for some assets. Criticality and failure consequences play a large role in that as well as the feasibility to inspect. Electrical devices for example follow random failure patterns and historically don't really have a timeframe between failure initiation and functional failure that's actionable, so a mitigation strategy I've seen done is something like hot spares if it's critical. On the other hand, something that is inspectable but won't result in high consequence of failure (death and injury are the things that are usually weighted heavily) it might not be worth inspecting either, it's all about trying to get the most out of limited maintenance budgets.