While the runway lengths seem alike, SNA has flat approaches at both ends and being a west coast airport, flights east do not have to contend with headwinds. Tweed apart from CLT and ATL, flights west to ORD and DTW would be subject to prevailing headwinds. Both HHH and EYW experience weight restrictions, HHH to CLT offers 65 seats on most flights out on the 76 seat E-175. Temperature, wind speed and direction, flight stage length and wet runways all mean at times more runway will be needed to depart with full flights and to avoid diversions. AA is doing well with the E-175's and with the tree coming down that will result in both runways offering 5600 feet, something that has never happened since the runway back in the 60's added 900 feet to the 4700 foot runway. If AA is doing well, why would not also DL and UA do well. The current plan is to pave 400 feet to the north and 1000 feet to the south. When one looks at HVN on an area map, there is a ready made market for air travel with a larger population base that BDL has among the tobacco fields. In the past, NW and SW surveyed HVN but the limited runway ended those plans. When Piedmont was dropping the Dash-8 for the ERJ-145, the 145 would not work at HVN, thus the CRJ-200 was the replacement for the Dash. I see UA coming in with CRJ-550 flights to ORD and DL with E-175 flights to DTW, hopefully next year. To say that Hartford has a robust market for air travel and New Haven does not is simply not true. Tweed still evokes memories of Dash-8's, B-1900's, Saab A-340's and Twin Otters, but passenger numbers are rising as word spreads that Tweed offers jet service. While 5600 feet can work under certain conditions, reliable service needs more runway to ensure completed flights under all conditions.
Is AA really doing well though? A Saturday only flight to CLT doesn't exactly indicate a top performing route. AA is using a plane that would otherwise be sitting on the ground in CLT, and (probably) spends the other six days of the week flying more profitable routes.
To me the CLT flight shows that airlines could serve other regional hubs (DTW, ORD, IAD, ATL, etc.) now if they wanted to; that is even more true with the obstruction removal and the longer usable runway length. But they're not doing it.
I don't think the market dynamics are really as much of a strength as you think. Yes, HVN is the middle of a densely populated area. But it is also sandwiched between two very well served markets, namely NYC and BDL. A lot of the population that HVN would want to draw from is in Fairfield County, which is very well served from a regional airport standpoint at HPN, and from LGA/JFK for longer range flights. All HVN would do for that market is duplicate options consumers already have at HPN. As you go further and further north of HVN, BDL becomes the preferred option. Yes, the area around HVN is densely populated, but with the Fairfield County and greater Hartford area markets spoken for, the only markets that would be significantly better served by HVN are the immediate New Haven area, and points east (and as you go east, the population density drops significantly). The market shrinks considerably once you take into account the segments of the population that are already pretty well served.
AA had a Saturday only flight from AVL to DFW for quite some time before it went daily. Saturday is the slowest travel day of the week and yet loads continue to be good. The tree just came down and the displaced threshold has not been moved back, they are awaiting FAA approval. You don't think HVN could support 15-20 or so daily flights on 65-76 RJ's? Just think of the thousands who who drive up I-91 to BDL who would use Tweed and the driving distance from Bridgeport to HPN is 43 miles, whereas HVN is 21 miles, less than half the miles and also avoiding one of I-95 busiest sections. For years people have been used to driving to BDL due to HVN service being sporadic, UA, DL and the perception of HVN being a prop airport. Now AA is using E-175's with a first class section, something HVN has not seen since 1996. I see Tweed's market going from Old Saybrook to Middletown and to Bridgeport and the many small towns in between. That constitutes a large market that can support many more flights. I see a great market and all it takes is for someone to try HVN and they will be hooked, nearby, no hassles and minutes from I-95 and I-91. Years back WN surveyed HVN but the limited runway ended those talks. Now with the advent of larger RJ's, Tweed can offer more service
I could in theory see around 15 flights supported by the market; the question is whether carriers have an appetite to serve that market via HVN, and I think they have shown that they really do not. DL could start service to DTW on a CR7 or E75 tomorrow if they wanted to, and UA could do the same to IAD. But they haven't done that in the past and nothing indicates to me that they will be clamoring to do so in the near future. Yes, it would be nice for the communities close to HVN to be able to have a shorter drive to the airport, but to your point they are
driving to BDL (or HPN, or LGA, etc.); the carriers are getting them as customers anyway without having to open up a new station at HVN (and incur the additional costs that doing so would entail).
Also, Bridgeport to HPN is 34 miles, per Google Maps, and Bridgeport is probably the extent of HVN's market reach to the west. From Fairfield down HPN becomes the more logical option. And I've tried HVN, and I was not "hooked" - 3 flights to PHL a day as your only commercial service is not going to be a viable option for a lot of travelers (myself included, especially when there are other options with far more service well within reach). And HVN's facilities makes HPN look...palatial (though if you only have 3 regional flights a day, they're appropriate).
I would love to see HVN grow and agree that there is a population around it that can be well served by increased service, but given the market dynamics and the facilities, I don't really see major growth any time in the near future.