There is a direct train service between Helsingborg and Stockholm (full route: Malmö C-Gothenburg C/Göteborg C-Stockholm C) with SJ X2000, although it's not frequent, once a day, departing Helsingborg early in the morning and arriving back at night. I believe there's an additional train on Saturdays during the summer months. But yes, flying is a lot faster
I'm all for sustainable travelling and feel that we need to cut down on emissions, but I'm not sitting 9 hours on a train to have a 2 hour meeting in Stockholm. From Halmstad (some of the more intelligent A-nutters may already have guessed that I live in Halmstad
) it takes 4:23 to get to Stockholm C by train (fastest option, otherwise times are between 4:45 and 5:30). The return with the same fast train give me 3:30 hours in Stockholm, and given local travel times, lunch etc., a 2 hour meeting is what is realistic. If I fly from HAD, I can take the ATR72 at 10:05 to BMA (no flights to ARN), and be there before the train. If I choose to go home late, the 20:40 departure brings me back home at 21:50. The train will cost ~1500 SEK and the flights ~2000 SEK (~200 €, these are the cheap fares). Halmstad certainly needs its airport. I'd be willing to go by train if the travel times were shorter, but as it is, we need to go by Gothenburg to get to Stockholm, and that is a time-consuming detour. Trains are not very fast either, lengthening the travel time further. There's also the issue with the unpredictable train schedules. There's a wild discussion in Sweden about what needs to be done, who's to blame and all that, but in reality, railroad maintenance has a 10-20 year backlog, causing all sorts of disruptions. If travelling abroad, we much prefer CPH over GOT due to travel options, both land and air.
Regarding AGH (Ängelholm-Helsingborg), I use that option from time to time, especially when I need to travel to destinations not available via Bromma (BMA). I then use SAS from AGH to ARN and on to LLA, KRN or some other place. Like most people in Halmstad, I have less than 10 minutes by car to HAD and 40 minutes to AGH. If HAD was to close, the best option would be to have airport coaches that were pre-booked and had a guarantee to arrive in time for each specific flight. I think that would appeal to a lot of people here. On the other hand, HAD is doing pretty well I think and the load factors are pretty good, even if the late return may be less dense.
AGH has a bigger catchment area than 200 000 people. I'd say 350 000 as Helsingborg and Ängelholm municipalities alone make for 190 000. Add the surrounding munis and 350' isn't that far off. If they decide to re-route the railway tracks so they can have a train station near AGH (walking distance, like ARN/CPH), that would really boost AGH. I think a lot of people from Malmö would consider AGH if that came true as travelling to MMX (Malmö/Sturup) isn't that easy, with a vulnerable and accident prone highway as the only realistic option. That would open up for travellers from all coastal cities from Varberg to Malmö, and all of a sudden, you have >1 M travellers in the catchment area. AGH also has a nice geographical situation as there are no cities really affected by the noise. From an environmental perspective, keeping AGH and closing MMX and HAD would make the most sense as travellers can go there by train (given a new station at AGH) or a shortish car ride.
On the east coast, RNB isn't going anywhere considering it's a military facility. Kalmar is 1:30 away by car and Kristianstad only one hour, so if KLR and KID closed, and RNB added capacity and increased frequency, that might be attractive anyway. On the other hand, closing KLR would leave large areas of Småland without air service and no really swift alternative to get to the capital. That would leave KID as the only candidate for closure. As Kristianstad is a regional capital with a lot of not-so-small cities nearby, I'm pretty sure they value their airport highly. KID is 91 % owned by Kristianstads kommun (municipality) and surrounding cities (9 %). I assume load factors are decent, otherwise BRA wouldn't be servicing the airport.
Inland, VXO connects people in the deep forests of Småland with the world. Roads are highways (often single lane) with 90 km/h limits and often only rare opportunities to overtake slower traffic. Travel times to other airports would be significant.
So, I think those "redundant" airports are there to stay, at least if no major infrastructure changes pop up.