Nice thinking.  We need more of this.  I did ask adam back about the long term incentives of bitcoin.  And he said fees might be the incentives, but,  "we have a long time to think about it".  I think you are right about 8-12 years.  The reward will be under 0.5 btc per block then.  I wonder what will happen to all the hashing equipment in that time.  It is wise to be thinking ahead.