A product roadmap is a plan, but a plan can be many things. At one extreme you have the kind of planning that just brought Philae to the Comet 67P/Churyumov-Gerasimenk where everything is calculated down to the smallest detail. At the other end is the plan for a soccer game, where you may not even know which players will be available due to chance injuries, penalties and much less how well they will play.
So, the real question is what KIND of plan should your roadmap be?
are you landing on a comet?
The “Comet-Landing” type of plans are meant for very predictable environments like outer space. Here, in virtual vacuum, all forces, objects and their trajectories are well known or possible to calculate. That is why you should calculate all the details that are possible and plan for them. You want to know at what time the satellite will be in what position relative to the comet and with which velocity. Before this you have measured the payload and documented every little detail down to the weight of every single fibre on the satellite (I even heard from a colleague who worked in the space industry that they once had a discussion about whether code could theoretically weigh anything).
are you playing a soccer game?
The “Soccer-game” type of plans are meant for very unpredictable environments. Thus a good plan cannot be too specific or detailed. you can’t plan that at 61.24 Ronaldo should make a cross with an angle of 47 degrees from 50 yards out to the right. It simply would not make sense. Instead you plan for a game concept, like possession based football, and find players that know and fit in that concept. You train situations that will reinforce that concept in pre-season and more specific ones related to the opponent the week before the match. That’s it: a concept and some generic patterns, that will be applied as we go along.
What type of plan to select
The difference between these two types of plans is the environment they should interact with. Both (the solar system and a soccer match) are dynamic systems, but they have very different levels of predictability. There is actually a mathematical term for this: Lyapunov time, the time it takes for a dynamical system to become unpredictable, or chaotic. For the solar system it is about 50 million years, the weather is 3 to 9 days and for a soccer game I would guess it is a few seconds.
So, obviously the key question in figurative terms is “what is the Lyapunov time of the environment your product is being developed into?”. That answer may very well differ quite a bit depending on industry and markets you serve. If you work in the space industry you inherit a longer Lyapunov time, since everything is more fixed and orderly. Contracts are made on long term basis and objectives are not changing that often (I am not saying they are not changing and the everything is very predictable. It is purely a comparative statement). If on the other hand you work in a start up in the social media space, chances are that you can barely predict what to do tomorrow and the Lyapunov time is very short.
If you are forced to plan 1 year ahead you should reflect on the nature of your product before you decide what level of detail and what type of plan you will do. Since scrum is from rugby, similar to soccer a flow type of game, it may make more sense to decide on a play concept and some vague patterns. You can and should probably plan something, just not in too much detail and with too much commitment.
I usually say that all my plans are back up plans in case nothing more interesting and pertinent happens. I live with a very small Lyapunov time.