We've had 20 guilds clear Solteris so far, another 3 at least will clear it this week once they get Commodus down. I am sure the server population is declining over time but overall it is still feeling great. I think by the end of the expansion we'll have just about double the "normal" number of guilds clear the expansion as compared to other TLP's in the Phinny+ era.
In comparison we had 27 guilds clear the previous expansion.
I think we'll see a steep decline for SoF clearing. Between it being new and harder content with tons of flagging requirement, and it being after the new TLP we should see some significant attrition even if Oakwynd looks to be a flub.