It's a trade-off. The example is simple enough that the alignment problem is really easy to see, but it also means that it is easy to shrug it off and say "duh, just the use obvious correct utility function for B".

Perhaps you could follow it up with an example with more complex mechanics (and or more complex goal for A) where the bad strategy for B is not so obvious. You then invite the reader to contemplate the difficulty of the alignment problem as the complexity approaches that of the real world.