High population systems should be much busier, agreed.
The relatively low traffic numbers we see presently could be because of the way NPCs are generated by each player's client.. NPC numbers are linked to the number of players in an instance, so the more players together in the same instance means more clients generating NPCs.
That probably explains why Sol or Achenar and other "big" systems have low apparent NPC traffic, its because the game isn't looking at system population stats to spawn NPCs, but rather is based on the number of players. If a system is full of players, it should fill up with NPCs too.
Might explain why Sol is so empty, since it doesn't tend to have much player traffic.
I think it would be much better if NPCs were spawned based on BGS stats instead: big population systems in Boom should be practically congested IMO.
I'd also like to see generally much bigger flows of NPCs moving between stations, as well as to and from inhabited planets; So many that the "shipping lanes" are easily visually apparent on scanners just from looking at the flow of ships.