2009 Chevy Duramax diesel.

There's no connection between using a fuel additive and the life of a glow plug.
Also, I'm not sure what he meant by the glow plugs "looked bad". The ends are always going to look bad after being in an environment like that. Most the time you cant tell a dead GP from a good one just by looking at it unless it's blistered.
Glow plugs are only used when the eng is cold. Generally speaking, after that they are shut down and do nothing. They dont act as spark plugs like many people think.

I grew up on a farm with diesel equipment. I was an over the road truck driver for a while. I've owned and driven diesel pick-ups since 2005. I've never used any additive ever.
My current '05 Duramax has 217,000 miles on it. The engine has had a water pump replaced and two injectors tightened up to stop leaks. That's all that's ever been done to the engine.

2007 Duramax LBZ. 250K miles no additives,no issues.

I have 140k on mine, I bought it at 37k. I used additives (white diesel kleen) in the winter when it is going to get super cold. And maybe one trip in the summer towing I'll go through a bottle of diesel kleen. I've had no issues.

I owned a Ford 6.0 for 35k miles and used one every tank, maybe the forums freaked me out more then anything in that case.

60k on mine. No issues. I use no additives.

Mine is a 2014 model (a bit newer), and has 30,000 miles. I don't use any kind of additive. Not sure what kind of shape mine will be in when it reaches 140,000 miles. But I would think, after 140,000 miles it might be OK to replace the glo plug.