the RFB is not convertible to multiple calibers
Sure it is -- put it down and pick up another gun of a different caliber! Think about it: with the RFB coming in ~US$1000 less than the MSRP price of the Desert Tech ... you can actually buy another gun of a different caliber ... and have two different firearms.
Also, a reminder -- you dial your rifle's scope in at one caliber ... then change calibers ... and have to dial it in for the second caliber. This is a solid use case for simply having two different rifles ... of two different calibers ... each of which is dialed in -- and just grabbing and going. (This, by the way, is my personal preference and approach, as well.)