Originally Posted by dabac

I used a 17 mm internal width rim on my MTB several years back, together with 2.1" tires. Handling was a non-issue. At the pressures I had to run to avoid snakebite, the bike tracked and steered entirely predictably. If I reduced pressure to get better "buoyancy" on soft surfaces I could possibly sense some vagueness when I hit a hard patch, but that'd usually be moments away from snake bite anyhow, a much bigger problem.
I've only had two tires pop off, ever. One was a clear case of overinflation(borrowed shop compressor), another was a bead failure for unknown reasons. Then of course I've had a couple of folding tires coming off as a result of a flat, but I think of that as another thing.

Biggest thing I can say about the combo I used is that it seemed to be extra harsh on the tire sidewalls, right where the bead merges into the sidewall. Had to retire several tires of different brands early b/c of fraying right there. But even that happened at an acceptable mileage.

In short: I think you should have what you originally ordered anyhow, if not for the actual benefit, so for your rights as a customer. Even if the narrower rim is unlikely to cause much of a problem(if any), it's still not the most suitable option.