I think Diesel raised this issue about a year or so ago, can't remember if it was behind the scenes or public. At that time I was moving towards the Triumph Trophy as my next bike going forward for a few years, given Honda had no intention of replacing the ST1300 with something updated a bit (not too hard as the basic platform is still great, in my opinion) My ST1100 is a 96 model with 213,000kms on it so it still has life left but might need more serious work at some stage if I kept of doing the rides I wanted. I decided to move to the Trophy as I wanted all the gear it had, plus it is just an easy and very sporty bike to ride, and just happens to be able to do distances as well. The ST1100 is staying though.
So after that, I am in the camp probably best expressed by Biggles - leave the forum and Australian ST specific, as this is just so much better than overseas based groups in many way. Rides, tech stuff, getting together etc etc can't be beaten.
As for other ST like bikes, if riders fit in or have to move on the other bikes for various reasons, including to a lighter bike that is easier to manage, I would see them still participating if they wish for all the reasons other than tech stuff - after all many have years of experience with the STs that would be a shame to loose. Sub-fora for other bikes are probably not necessary, as the current rides, events etc fora will cover that off, with technical stuff available on bike specific fora..
I have a few trophy owners who might like to do the ride and social aspects, but most I know of in Australia have not come off Honda STs so would not have that 'common' bond with the bulk of members (whether current or past ST owners). Overseas, there are many coming off ST11s, ST13s, BMW RTs, BMW1600 etc etc. But my experience with the best Trophy forum out there is that the owners have much in common with Ozstoc in terms of maturity, getting on and sharing.
So to cut it short, evolution as we go along but with a Honda ST heart. Keeping to that core makes managing things easier and more focused on what we are about, so less chance of loosing direction in future.
I must add that I still have my Suzuki 1978 GS1000, and there is still a very active forum with GS models even given their age. And in the last few years the BMWs of old are coming out of the shed in droves, so there is a lot of life in bike specific fora, especially given our links with world wide owners.