I've never owned or run an USBPre, but I currently own a D50 and a V3, and have previously owned and used the SD 722 and 702.
I certainly wouldn't dispute that running a single 702 isn't the more rugged solution than a USBPre + D50 coupled together with an optical cable. That said, I've run V3>optical>D50 for years and never have had any problems. With an adequate gear bag (I use several different LowePro bags) and a good optical cable -- very much recommend right-angle on both ends cable to reduce stress -- you shouldn't have a problem.
If you have a D50, adding a USBPre costs another $600. Assuming you can sell the D50 for $350 and buy a 702 for $1550, going the 702 route is $1200 out of pocket, or twice the cost of the USBPre+D50 route. From everything I've heard, I'd say the sound would be close to exactly the same. Clearly you don't want to lose important recordings due to gear failure, but my experience has been that being moderately careful (I drink bourbon and beer while recording, can't say I've been anything more than moderately careful), you'd be fine.
Another option: I ran a V3 for 6-7 years, several with a D50, and then decided to change things up and get rid of the V3 and use a different preamp for awhile. With the USBPre coming out, I thought long and hard about getting one. Then I realized for another $250 or so more than a new USBPRE, I could get a used V3 with the optical out.
I decided on that route -- used V3 + your current D50 is less $ than a used 702. I ran V3>722 and V3>702 for a few years. I've run both V3(analog)>7xx and V3(digital)>7xx, and have run the 7xx as a standalone -- all lots of times. I finally decided I liked V3(analog)>7xx best, but I liked V3(digital)>7xx or D50 much better than a 7xx standalone. My ears and ymmv, but I think a V3(optical)>D50 rig sounds noticeably better than standalone 7xx, and in your case, noticeably cleaner and more transparent (for the nature recordings).