Has anyone mentioned off-chain transactions yet? 

My guess is that they're not likely to be a problem for at least 6 reward halvings (ie: 24 years) but after 14 halvings (ie:56 years)  I would expect them to have already started impacting the hash rate and thus blockchain security in a major way.  That is, somewhere between 6 and 14 halvings total payout (block reward + transaction fees) in equivalent fiat at today's dollar value for mining a block will peek and start decling thus causing a peek and decline in mining.