I believe TG is fully aware of the challenges in this price structure. CPH/ARN to BKK are weak premium markets which makes it difficult to sell C but the PE cabin is an attractive product for strict company travel policies and upscale leasure travellers.
TG has been offering PE to Scandinavia since 2003 but the product has never really been highlighted by TG (only the A345 PE product was ever mentioned on the webpage). Still it has been selling well and apparently the product is so strong that TG is ready to reserve half of the C-cabin for PE from next summer.
True, other airlines offer strong C-fares from time to time but they don't offer the flexibility of the TG U-fare and for destionations like HAN, SGN, CGK, KUL, MNL etc. TG offer strong connections. Only time will tell if TG is able to keep the same fares with the 77W.