A few thoughts:
I believe that if people want a secure network, they will figure out a way of getting it. My justification is the same as my belief that if people want clean, cheap, safe drinking water they will figure out a way of getting it.
Both conventional wisdom and economic theory have been called into question recently by a series of research papers which report experimental studies of collective decision-making about public goods. Almost without exception, these papers have reported results that cast serious doubt upon the importance - and, in some cases, even upon the very existence - of the free rider problem.
The people who are most likely to care about security are the people with a ton of stake in the system. The average person, having a few thousand dollars worth of coins (or less), is unlikely to want to fund miner security if they know that there are already tens of thousands of dollars of hashing being done every day. They will probably be a collective deadweight, simply because of a diffused sense of responsibility. (10 million people each having $1000 stake will not fund as much mining collectively as a single person with $10 billion stake). Enough of the heavy hitters though will care to fund the network to a reasonable amount of security. I think this will happen at any value. If a bitcoin is $25, the heavy hitters will have a realistic sense of what the hash rate needs to be to protect them. If a bitcoin is $250,000, the heavy hitters are now probably governments and corporations, but they will still have a realistic sense of what is needed to properly protect their stake into the currency.
I think that altchains and altcoins will be able to play a big role in this. If permanent inflation ends up being required to encourage large amounts of hashing power, the heavy hitters will switch to a chain that has permanent inflation built in. If % fees on transactions end up being the most attractive choice, then the heavy hitters will switch to the chain that has % fees, because though they may not like the fee, they value the security and the heavy hitters
will care enough to stomach the fees. If a really restrictive block size ends up being the correct choice... and so on. Sidechains could let all of these experiments happen on the Bitcoin currency directly.
My personal guess is that a permanently inflationary currency is going to be the long term solution. I don't see assurance contracts panning out, I don't see the value in restricting the amount of transactions (beyond enabling the average home connection to keep up), and I don't think people will voluntarily pay higher than necessary fees.