After 20-50 years even though people can still mine and get a lot of coins, the inflation will be really small because the amount of coins in existence will always increase against the amount of coins mined. Because of this there is really no reason to cap or slow down the supply after a set amount of years. The volume of coins over the new coins will always increase thus decreasing inflation.
After a hundreds years inflation would be 0.0001% for example, but even this would be enough to keep people interested.
I really like this idea. If you add a cap then I think you will miss out on something important.
While the forward thinking is awesome, don't you think people would get kind of weirded out if they used to mine with 100 SQC (yes, I'm using my own name) rewards and suddenly everyone with a CPU is getting 1000 SQR a block?
I don't think perminant exponential reward growth would help the coin to be adopted and used to that it's relevent for that long, even if it's mathematically ideal