This thread started two years ago, but the topic remains topical. Not just for big band concerts, musically very different community groups - Scottish Fiddlers, classical, old-time jams, experience the same. My experience is with traditional acoustic music styles, including organizing one of the longest running banjo and fiddle festivals in the US. This is on the history, but I also wrote articles on staying relevant:
https://folkworks.org/60-years-later-th ... ntestants/
How do you keep/grow audiences? The focus needs to be on the audience who at the minimum gives their time (and more often than not also money). The event needs to be attractive, good music helps, but it is just one of many components. Pleasant locations (or crammed seats in a stuffy room), do you meet people/friends before/after, new contacts, performers that care about the audience (rather than themselves)?
From the first posts, the photos may explain the declining audience regardless of music style. The group photo feels like "watch grandpa and his friend play bingo at their retirement community". Audience pull is going to be minor and would be the same whether that group tried to rap, shoot hoops, tell corny jokes, or play big band charts. Friends and maybe some next generation family.
Second, we see a rather uniform age group. All old men whose circle of acquaintances isn't growing. You want kids from the local school to come? Families with children (who aren't your grandchildren)? Start with having some in the band or as featured soloists, one from each of your local schools and you'll see a lot more younger people filling seats.
Community groups with a better staying power have that generational mix and keep renewing. As do successful festivals (I follow my own advice and turned the booking for the Topanga Festival over to somebody 25 years younger who has the social circle I had then). One of the most successful organizations in that respect is the California Bluegrass Association, whose events impress me every time by how evenly age groups and male/female musicians are represented.
Broaden the appeal of the group (socially, the music could very well stay the same, the kids in the California Bluegrass Association all play the 1940s/50 Monroe/Flatt and Scruggs classics) and the seats will start filling.