I fear you're correct. As an ex teacher in primary schools, I've also seen a marked swing away from brass lessons either as a class or in small groups like I once used to see. Now, it's all guitars or, worse, Ukeleles, and keyboards.
It's bound to feed through in 10 years.
Playing Brass has given me a lot of pleasure, both as a teenager and as a returning adult; Trombone and Bass have been near perfect instruments for me to grow as a player but they wouldn’t be right for everyone and indeed Brass isn’t right for everyone either. Getting people playing music is what matters and, of course, once they are playing then they are building complementary and transferable skills. One of the most gifted Cornet players I know came from a Guitar playing background and took up Piano - which he was also very skilled on - to inform his Brass and Guitar playing. It’s also usual for MD’s to be able Pianists as well as Brass players.
For some reason Brass Instruments are expensive yet Ukes, Guitars and Keyboards can be both very reasonably priced and more tolerant of poor care than Brass. Brass instruments - particularly Trombones and Tubas - can also be overly large and difficult for primary school age children to hold, operate and move around. Ukes, Keyboards and Guitars use chords in producing music, are helpfully quieter than Brass and can accompany singing; they arguably are better music teaching instruments than Brass and to my mind teaching and motivation to play must come before historic choices of instrument.
It’s easy to say that Brass is best and all else is second rate or worse. However the most successful professional musicians don’t play Brass, they play Keyboard and Guitar. The Beatles also played Uke too, they used the Uke as both a performance instrument and one on which to develop songs and tunes prior to moving them onto performance on the Guitar. If logic is applied then I find it impossible to look at Guitars, Keyboards and Ukes as worse choices than Brass instruments.
Brass, of course, is what we love and Brass playing in a group can produce fantastic music. My own favourite Brass group is the Quintet rather than Band and contesting leaves me cold, but others love contesting and for some folk anything less than a full Band is only part complete. We’re all a little different and I think it best for children - and adults too - to be encouraged to play an instrument without thought of much more than: what brings them joy; who’s actually available to teach students; what’s affordable; and what’s educational?
What will feed through in ten - or other - years time? Well nothing if we fail to get the young playing some musical instruments and possibly something if we do. The something will be able to read music, have some music theory knowledge and have some idea of playing music with others. At that point it’s up to us as Bandsmen and Bandswomen to attract and persuade people to give Brass a try, people who can already play an instrument well are usually able to pick up Brass quite quickly and they bring with them welcome knowledge and attitudes.
I can remember being 51 like it were yesterday . . . an' killing a woolly mammoth to celebrate the tribe learning how to make fire, instead of just sitting around waiting for a thunderbolt to hit a tree . . .