99.21875% will be mined in 20 years time. If electricity isn't very cheap by then and/or if bitcoin isn't worth hundreds of thousands of dollars, it's hard to imagine anybody wanting to mine then. How will transactions then be verified?
You're ignoring at least three important factors:
1) The total amount of transaction fees: In 20 years - provided that Bitcoin is successful - a lot more transactions will be processed by the network. The average blocksize will most likely be bigger than today. That means, miners will obtain much more fees than today if they successfully solve a block. This gradual transition from a block reward to a fee reward operating mode is part of the original conception of Bitcoin.
2) The valuation of Bitcoin: If Bitcoin is successful, a single coin will most certainly have a much higher buying power than today. Hence it might be still very profitable to mine given electricity prices are not appreciating at the same rate.
3) The efficiency of mining: Bitcoin mining is under constant optimization to lower costs and improving profits by increasing its efficiency. 20 years from now, Bitcoin miners will not only mine faster and with less electricity than today, it is also thinkable that "by-products" of mining - especially heat - are utilized for other purposes and monetized to increase profitability further.
So I don't think that there is any reason to worry that mining might end in 20 years.
ya.ya.yo!