That line of argument absolutely ignores competition at these non-served airports, their largest markets and relevant WN presence in desired destination cities, and the role of proximate other-carrier hubs. This is single-vector route planning - nothing but airport total passenger numbers. We might as well turn route planning over to a bunch of elementary school kids with no knowledge of markets, geography, or statistics.
The number of domestic passengers that SYR carried in 2019 was similar to that of GSP or LIT in 2019. AA and UA also serve more of their hubs nonstop from GSP than they do from SYR, with AA serving DFW nonstop from GSP but not from SYR and with UA serving IAH nonstop from GSP but not from SYR. B6 also doesn't currently serve GSP or LIT, whereas B6 already serves SYR.
AA and UA also serve more of their hubs nonstop from GSP, with AA serving DFW nonstop from GSP but not SYR and with UA serving IAH nonstop from GSP but not from SYR.
WN also already offers 1-stop connecting service to some destinations not served by B6 from GSP and LIT, and WN would be offering 1-stop connecting service to some markets not served by B6 if it adds service to SYR.
GSO and TYS are also both located in two of the largest U.S. metropolitan areas (by CSA population) without WN service. Dayton (OH), Chattanooga, and Harrisburg (PA) are the only other U.S. metropolitan areas with a population of 1,000,000 people (by CSA population) that do not have WN service, but DAY, CHA, and MDT are near other metro areas and airports that have WN service.
While AA serves its CLT, ORD, DFW, PHL, and DCA hubs nonstop from both GSO and RDU, DL and UA serve fewer hubs nonstop from GSO with DL having nonstop service to BOS, LAX, JFK, SLC, SEA from RDU but not GSO and with UA having nonstop service to DEN and IAH from RDU but not GSO. RDU is also served by some airlines that do not serve GSO such as WN, B6, F9, and AS. There is also some leakage to RDU from the Piedmont Triad region with RDU having significantly more options than GSO, even on the airlines that serve both GSO and RDU.
GSO might be able to support WN service, despite the proximity of RDU to the Piedmont Triad region, due to
(a) GSO being one of the top remaining contiguous U.S. markets (by both metropolitan area population and number of domestic passengers per year) without WN service,
(b) there being some leakage to RDU from the GSO market with RDU having significantly more options than GSO,
(c) DL and UA serving fewer hubs nonstop from GSO, and
(d) WN capturing some of the passengers who would otherwise fly out of RDU if it adds service out of GSO.
Even though GSP is only 118 miles great circle distance from TYS, the driving distance to Downtown Knoxville from GSP is 183 miles due to the Appalachian Mountains. BNA is closer driving-distance wise from Downtown Knoxville than GSP is with BNA being 174 miles (by driving distance) from Downtown Knoxville, plus BNA has a much bigger WN presence than GSP does. TYS is far enough (by driving distance) from BNA, GSP, ATL, and CLT to support WN service out of TYS, whereas most of the other top remaining contiguous U.S. airports without WN service are closer (by driving distance) to other airports with WN service.