Ahh yes - if you're only flying domestically, then that's right I believe - you won't be able to earn UXP due to the issue you mentioned. For someone who flies only or majority domestic routes, Flying Blue might not be the best option due to the paltry 2xp 6xp in domestic F (coded as J) per segment.
On short domestic F/J segments (less than something like 1000 miles, depending on targeting Gold or Platinum... if not going west of the Rockies often, it's not difficult to take a DTW/MSP/ATL/LGA connection and keep the average length well below that), there's a gulf between DL elite credit and FB elite credit. 50 segments of 1000 mile distance will get you FB or DL Plat (assuming you meet the the MQD requirement, which would mean $12k or $240/segment... $25k of credit card spend would eliminate that requirement, but $15k on the FB card (which is easier to rack up big spend on...) cuts the segment requirement (at $30k on the DL Reserve, you'd get a similar ratio reduction in flying through the MQM boost)).