Discovered this after looking at the duplexing unit. I think it's cooled toner. It does seem to leave a slight residue on single sided printing. Barely evident.
Since the repair person actually saw the fuser I would take his word over someone on the Internet, and he would not have been referencing it by page count, he would have been judging by the roller itself and experience with them.
The page count is the expected under ideal circumstance, but it won't make it even close if the end user is loading odd stocks improperly set for weight. So if you have it set to plain paper and load cardstock, the fuser takes damage. And if you have the device in a humid environment, or in a cold area, it can cause fusers to wear out faster than under the ideal.
Your page count does seem extreme to be fair, but if the repair person warned you upfront of a future failure, there had to have been a reason, so it would be much more beneficial to just contact them again to ask what they saw and what they think may be the cause so it can be avoided in the future.
And all that is not to say the fuser is the cause though, the fuser gets used whether or not a job is duplex, If it is just a duplex issue it is quite likely that there is toner contamination in the duplex unit, else it would be appearing on simplex jobs as well.
A few months ago we had issues with having c/m/y/b lines appearing across the page and had a repairman fix it. He mentioned that the fuser would be needing replaced soon. As you can see I get some streaking when I duplex. I was going to replace the fuser and found on the parts page. It states "long-life item, typically not required", with a capacity of 50,000. We've only printed a little of 11,000 pages, so I'm wondering if this is actually the issue. Any advice would be appreciated. Thanks!