I would say that the Solo and FF15 fleets have been consistent over the years I have been there but the handicap fleet is well attended and with a decent level of competition. If I was lighter I might consider a Solo but I am not and will never be so I am very happy to race in the handicap fleet.

Our biggest fleet is RS400, Followed by Aero's (but is that 3 fleets as the rigs vary?) then, non Asymmetric handicaps and 2000 which this year seems to have taken a drop for some reason.

400 are getting between 8 and 15 on a Tuesday night and 6 to 12 Saturdays

My local club doesnt really react. The only changes to official fleet status in recent memory were Phantoms gaining then losing it (once the PY came down to a realistic number the pot hunter left) and the venerable Enterprise finally losing fleet status after years of no turnouts.

People are pushing for the D-Zero to get fleet status (as they dont like us winning consistently) but we just dont have the turnouts at the moment.

We have faced pressure in the past to add new fleets and typically it seems this comes when six boats of one class are regularly racing but we have resisted this because all too often things are fluid and people change class or clubs from season to season and we plan our sailing schedule in September for the next year so its hard to predict.

Personally I think that the reality of dinghy sailing will see most clubs head toward a general handicap fleet with people that want to fleet race heading to opens to do so. I am not saying that is optimal but it reflects the reality of our sport.

If all they wanted were separate results then that shouldn't be too hard to arrange, potentially even ex-post based on the number of boats that actually turn up. This is certainly how we do it at our Junior Race Week.

Same here and it does annoy me when the RO is lazy and chucks the fleets together so you end up racing boats that are in a different race to you.

Our FF fleet goes off first, then handicap after 3 mins and solos last 3 mins later. In any kind of breeze I easily catch most of the FFs within 30 minutes and they get very grumpy that this wipper-snapper in his fancy carbon boat is going over the top of them and they shout at me for interfering with their race. Lost count of how many times I have had to point out that I am racing too!

But is the inverse true? If clubs leave fleets to dwindle to 1-2 boat turnouts then does it have no effect? Or does it make people more likely to decide to move classes / stop sailing? 

I wonder if it is easier to move classes within a handicap fleet where (assuming a not too large PY range) the style of sailing remains broadly similar and the social group remains the same. 

Moving between  OD classesis perhaps more of an upheaval (different social group and maybe significantly different type of sailing/ownership/maintenance) which might make people more inclined to just stop sailing?

Very nicely put, and with this mindset you can sail an old uncompetitive boat and still enjoy. I have always targeted sailors, regardless of class, to beat on the water as my goal.