I get it that language changes and that those who resist change are in a futile fight. What I don't like is when we lose a chunk of meaning, which is what happens when a word that has meant a certain thing loses its firm connection with that thing and there is no ready substitute.
For one example, heroism used to mean actions that were extraordinary, specifically actions above and beyond the call of duty, and which involved some sacrifice, such as a risk to life and limb. That's what got you the medal. Just doing your duty, without that extraordinary element was not considered heroism--it was just doing your duty. Heroism has devolved now to mean merely brave or, worse, simply famous, as in "sports hero." Thus a cop who confronts an armed robber and captures him is not a hero. That's what we pay him to do, nor is the fireman a hero who goes into a burning house to rescue a child. We would condemn him if he did not. The policeman who goes into the burning house and the fireman who stops the robber are heroes, but we don't have a word for these deeds that would distinguish them from the expected bravery of duty in dangerous occupations. This is a real loss.
Another loss is the idea of tragedy. For a long time tragedy was a literary term denoting a certain kind of dramatic plot, in which a noble protagonist destroyed his life through some flaw in his personality. This sort of fall is a common observation in daily life, and so the term was extended to include such events--the decent man who can't control his temper and kills his wife, and so on. But when "tragedy" is made to include any sad event, this specific usage is lost, and we hear of the "tragic" loss of life in a natural disaster or in the accidental death of a child. Thus we can't distinguish the sinking of the Titanic or the Gulf oil spill, which were real tragedies, resulting from human pride and folly, from an earthquake or a tornado, which are mere catastrophes. We may even lose the ability to understand actual tragedy and what it signifies to the human condition--another real loss.
I suppose this has always happened with words, but I guess I am now old enough to have lived through significant changes and to feel them happen in my head. I was sorry to lose gay as meaning light-hearted or fun-loving, and this one has the added difficulty of rendering a large number of songs and poems ludicrous or confusing--gay was a fairly common word before its slang usage to denote a homosexual overwhelmed the common usage.
It would be nice to think that new words would fill in or substitute for the meanings the old ones have lost, but this is not always the case. We actually stop being able to say certain things in English in as convenient a way as we once did. An example that sticks in my mind: the word "modern" in Shakespeare's time meant an absurdly exaggerated attention to dress, especially where associated with social climbing. When that sense faded before the current one--referring to the present age or to being up-to-date--English lost this useful descriptor, and it has not been replaced, except by such adaptations as nouveau riche, which are not exact.
Now back to worrying about global warming.