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TBF I have 2.1 WTB Nanos in 17mm internal Mavics on the GF pootle wagon. The tire shape is good and the thread is in the right place.
Probably a more dramatic effect in road wheels where we use 25mm tires, 5mm wider rim is huge. For making an appreciable difference in the shape of a 2.1 tire you'll need a big jump in width, and if you're using rim brakes it's limited by how far back you can push the brake pads and availability.
As Netakure suggests, the original MTBs used pretty skinny rims. IIRC, Mavic G40 was the rim of choice for the 29er originators, and they're only 15mm wide. A 2.1" tyre on a 17mm rim won't be lovely by modern standards, but it will only be as horrible as what we all had in the 90s 🙂