I imagine that at that point, Bitcoin is going to be very expensive. Transaction fees are going to be the incentive that much is clear, but the question really becomes how do we find a balance between the price of those fees to keep Bitcoin in use and miners happy. The fees will have to be lowered for the price to keep increasing, but miners aren't wanting to budge on that either. Right now it's too expensive to send Bitcoin already in my opinion and that's why I use STEEM or LTC to transfer funds. I don't see a long term "super high" price without lowering transaction fees, but who knows what the future holds. That's just my two cents.