Currently, every day, each player above 10000 influence points in a port, loses 0.5% of his total influence in that port.
Details of this decision can be found here:
viewtopic.php?f=5&t=179If you read up that topic, the 0.5% was inserted as a starting point which would be reviewed later. We may increase this to 1%. We first want to launch the titles feature. Then, according to the way people hunt influence after this point, we will be deciding if it will be needed.
1% may sound like still small but it is not. Practically, it will double the current difficulty to keep a port. Influence deterioration is very important for the game balance as it acts as a gold sinker. The current imbalance seems to come from the use of Charity Donation card. This is why we created conspiracy card.
Another important factor is that many nation missions for the titles will require the players to help influence of their nation. Therefore we will get more people involved in the influence hunt. This alone may prove enough as currently, not many people bother with it. Even if they are winning millions per day they still don't spend a penny for influence.
Once more people start to bother with influence, Charity donation will become more rare and important than it is now. This will also might favor to maintain port control with gold.
One more important factor to keep in mind is Propaganda card. Charity donation has certainly been used more than Propaganda so more Propaganda cards should exist right now (I have not checked the database, just common sense). Propaganda is the counter-card of Charity donation. This means that in a probable war, propaganda could vanish a port influence completely.
If you want a verdict, in the future, it will be counter-productive for any nation to hold more than 2-3 ports. This could happen only due to the increased competition for influence. Every nation will need/want at least 1 port and there are only 21 ports for about 200 nations...