I don't know where you get the idea that they 'protect' it unless you're suggesting that they introduce a cheaper 'bang for buck' model to compete with Uni-T for your hobbyist dollar.
The renaming of the 87 IV to the 189 was clearly done to protect the traditional 87III market at the time and avoid confusion that Fluke were going to discontinue the 87V form factor and features.
It's likely that no amunt of reassurance that they weren't going to discontinue the 87III worked, so they changed it quick smart.
And then it's been the same design and features for two decades now, with no visible upgrade. The 87V came out in 2004, almost 20 years ago now.
In that time a lot has changed inside, IICR they switched processors to the MSP430 and did other things, but outside it was the same.
They are clearly protecting a winner, and Fluke have told me as much.