I think it is absolutely appropriate/fair and good to use the term "former" World Champion to describe someone's accomplishments.
1/ It is good for the readers to know and identify whom the current champion is. For example, without the term "former", it would be confusing for a lot of people to know who the current US Open Champion is, who the current world's 9-ball champion is, who the current world's 8-ball champion is, etc...
2/ The term "former" World Champion should not be taken as a slight. Anyone that has their names attached with the word champion, or "former" world champion, should be very proud. It is an accomplishment that cannot be taken away from them. Which is the reason why the writers associate their names with the term "former" World Champion.
3/ Past accomplishments do not equal current accomplishments. Yet, past accomplishments should be identified. Which is why the term "former" is very appropriate.
4/ As you said, once a World Champion is always a World Champion. But he/she is not the CURRENT World Champion.