I think you get a better idea off the telly than being there. In years gone by I've often felt the Holte was full, only to watch match of the day and see swathes of empty seats ahead of me. On TV the lower Trinity looked literally half empty and the ground sounded half empty when we were singing and the overall atmosphere. I don't think 30,000 is a low estimate.
Like some others I pick and choose my midweek games travelling from distance, others have work or childcare commitments that keep them away in the week. I pass my ST on so my seat is still occupied.
I have to say that CL pricing has made me feel less inclined to put myself out for any midweek game.
However, to make anyone wanting to sell their seat, wait til it's sold out is short sighted by the club. People will be looking for pairs and groups of tickets, or to be in a particular stand where friends/family already sit & may buy resales where they wouldn't buy singles or GA+.
The club still profit on resold ST seats as they refund the 1/19 t price less an admin fee and can resell at full price, and have more people in the ground buying catering and merchandise.
Overall it's a poor strategy from the club.