The likelihood of your 50W router suddenly pulling between 16A and 20A is pretty much zero. If it ever fails in an overcurrent situation, it will likely fail catastrophically (short circuit) and trip the breaker.
The problem is future owners - and even future you a few years from now when you've replaced the router and moved it to another location. And the HVAC quits, and this room is now the place you want to be warm and cozy (it might not be that room now, but room uses change over time) and you plug in a heater. Or worse, 2 heaters. And each heater draws 12A (the standard amount), for a total of 24A. Which is 20% above the rating of a 20A breaker. Which means that depending on the breaker and the trip curve, it may take a while to trip.
In fact, looking at a sample trip curve from Schneider Electric, 20% overcurrent might trip in 15 minutes or might never trip, but your 14 AWG wire could eventually overheat and start a fire. On a 15A breaker it would be 60% above the rating and trip between 2 and 15 minutes, preventing the 14 AWG wire from burning up.