One of the Product Owner’s key responsibilities is to ensure the backlog (well, the top of it, at least) reflects the stakeholders’ priorities. Of course, the certainty of those priorities decreases the further out you try to look: we ought to be pretty confident in the priorities for the next couple of weeks, but less so for a couple of months away, and if we’re thinking 6 months ahead then it’s quite a low certainty. (Your timescales may be different but the “funnel” model should still be applicable.)
The reason certainty decreases the further into the future you try to predict is that things change! Hopefully, you’re learning more about your customers over time, but also their needs change. There are also changes in technology which mean something you couldn’t build last year is now possible, or maybe the costs have dropped so now it’s worthwhile.
Like everything we do in Agile, it’s about feedback: we think the customer needs X so let’s build just enough so that we can test the market, i.e. a Minimal Viable Product. Based on the learning from that we should adjust the backlog, so putting too much effort into the details of a plan that’s likely to change is waste. No matter how much you believe you know what your customers need and spend time drawing up a 12-month plan, if your plan doesn’t change then either (a) you’re a genius, (b) you’re really lucky and should play the lottery, or (c) you’re not listening to your customers.
Even in a tightly regulated market, things change – if your product is constrained by legislation, there can be changes resulting from interpretations of the law or even changes in the law itself. If your plan is unable to reflect those changes in a timely manner, don’t be surprised when someone beats you to the release of a “fully compliant” version.
The key is to have a plan which gives enough context for the current/imminent work but which is flexible enough to react to changing information, as well as to have people who understand that a plan could (and should) change when circumstances change.
Maybe that’s the most important piece: having people who ask why when a plan doesn’t change?