Demurrage is exactly the same thing as increasing the money supply. There are no special economic results that will occur under demurrage that will not happen under regular money supply inflation.
Demurrage is simply expressing the amount of bitcoins you have as a fraction of the tot6al supply instead of hard units.
This is key. Just to explain a bit more, let's say that in 2010 there are 10 million X in existence and you have 100 X. In 2020, the number of X goes up to 20 million. The result is twofold:
1) Your share of the total sum of money has decreased from 10 per million to 5 per million.
2) Every 10 minutes (assuming money supply growth is exponential), a miner gets coins equal to 1.3 per million (since 1.0000013 ^ (6 * 24 * 365 * 10) = 2). This income transfer to miners carries certain economic effects (encouraging mining, reducing the bitcoin price due to for-profit miners wanting to sell, etc)
Now, let's say there is no inflation, but there is demurrage. In 2010, there are 10 million X in existence and you have 100 X. In 2020, there are still 10 million X but you have 50 X. The result is:
1) Your share of the total sum of money has decreased from 10 per million to 5 per million. In order to effect this change, every 10 minutes, 1.3 per million must be lost out of every pool of money every 10 minutes (since 0.9999987 ^ (6 * 24 * 365 * 10) = 0.5).
2) To keep the money supply constant, every 10 minutes a miner gets coins equal to 1.3 per million of the total money supply.
The two scenarios are EXACTLY THE SAME.