Less than 1% over 4.6 billion years, at that proximity to the sun, ain't that bad. The sun is expected to swallow Mercury in about 5 billion years, so it's going to meet a violent end long before it evaporates away into nothingness.
Why do they say these things - when most people havent a clue or opinion if or why Mercury is shrinking.
(Yes, the energy released by that process would quickly get very obvious. If you've got a passable hand-wave, audiences are very forgiving of plot holes in SciFi.)