As you progress in skill with one field or another, it's often important to ensure your other talents don't rust away.
I write on this site daily and often cover WordPress as it's highly relevant to my day job. Ironically, when I don't cover WordPress, my traffic goes down.
Likewise, when major WordPress events (i.e. specific WordCamps) hit the schedule, my traffic goes down considerably. This is because the majority of my readers shift their focus from my articles (which occasionally don't cover WordPress at all) to the WordCamp event of the weekend.
My readership yesterday, for example, dropped by 60% - while my Twitter feed spun out of control.
Expand your Audience
My skills aren't limited to WordPress. I write about physics. I write about religion. I write about business. I write about writing.
Unfortunately I don't talk often enough about my work outside of WordPress, so my readers don't follow it - or I don't have enough non-WordPress readers to make a significant difference.
For this site to remain successful, that must change.
For your site to remain successful, you need to learn this lesson as well.
What are your hobbies outside of work - write about them. What are your other interests - write about them. What ancillary skills are needed in your job that aren't in your job description - write about them.
There's an audience for every topic. The trick is to be broader than a one-topic site. Being an expert in your field requires knowledge above and beyond your primary responsibility - leverage that knowledge to expand the content of your pillar articles and expand your audience at the same time.