What’s wrong with it?...........
“one of the only remaining”
Best is to get the actual number (thus the comment about a research assistant if he doesn’t want to do his own research) - ‘one of x remaining’ or even ‘one of only # remaining’ or perhaps leaving # remaining.
Next best - ‘one of the few remaining’.
‘the only’ isn’t proper grammar. Of course, there have always been reporters that were poor writers. There used to be copy editors that made them look good.
Just because poor grammar is becoming more common, it isn’t becoming correct. See
https://brians.wsu.edu/2016/05/19/one-of-the-only/