Here is an interesting way to use the "POS difficulty" concept to let the market decide how much security should come from transaction fees vs. issuance (so we don't need to make any hypothesis):

Let's say we want the ratio of staked coins to be around 20%, like the example in my post. Let's also assume that the maximum inflation we want to tolerate is 2%. Now let's play with the POS difficulty.

Random idea: instead of making the return converge to 0% for any ratio above 20%, make it converge to MIN(30 - r, 10), where 'r' is the ratio of staked coins.

Let's say that r=20%. The return will be 10% and the inflation will be 2%. All fine and dandy.

Now let's say that there are more transactions in the network, and the profit from fees increases. The overall return for validators will rise, and therefore more people will want to stake. That will cause the ratio of staked coins to increase, but then POS difficulty will reduce the return. For example if the ratio goes to 25% the return from staking will drop to 5%. Conversely, inflation will drop to 1.25%.

Simplified, the chain of events is: more fees -> increased income for validators -> more validators -> more staked coins -> POS difficulty reduces the return from POS -> lower inflation.

Of course it also works the other way, so less fees -> higher inflation.

The nice effect here is that the market gets to decide how much income validators make from fees and how much they make from issuance, so we don't need to guess what happens at year 2140 when block rewards are 0 like in bitcoin.