I don't see a problem with payed aircraft addons effecting on-line play other than the aircraft not being available to fly on the particular server you like to frequent. The server admins select the aircraft available to the player that best supports the game they are trying to achieve. It may or may not be a bit biased but its their server and their game. If it doesn't suit your needs, join another, or create another server.

Who could get bored with the Battle of Britain? Flying in at altitude in your Me 109e, depending how far you fly in you have between twenty and ten minutes combat time before you must break for home :)
Awesome stuff.
Flying ADW I sometimes spend ten to twenty minutes stalking my prey... Every minute is worth it as you dive down and squeeze the trigger.

But thats me!

Yes others will scream for more but like most I believe with the italian planeset already in SOW BoB a mediteranian campaign/added on will come very quickly.
My guess is shortly after that will be operation barbarosa, early Eastern front and Finnish winter war.
Planeset will not have changed much from BoB, just newer spits and the early Russian stuff in barbarosa and the F series Me 109 for the huns.

As Oleg himself said he will open up the game to modders (only smaller maps from the begining) and only keep the bigger maps for SOW add-ons.
I expect them to turn out an add-on a year. Add to that the Korean project working forward and a possible WW1 working forward as well and we are in for a hell of a ride.

So maybe those that will be bored are those that really don't deserve SOW, but then they never hang around long anyway.

Well Oleg did make some cryptic comment somewhere about after SOW there would be a theater not yet covered. But .. no guarantees .. be sure.