Even then, the World Cup is a different beast to the Asian Cup. Maybe Bird's Nest at a stretch, but I'd expect to see rectangular stadiums otherwise.
I agree, they have the money and the resources to build brand new ones, and in very quick time if needed. I have a feeling that the venues being built for the Asian Cup are a stepping stone to a WC bid because there isn't really any other reason to build them - all the nominated cities have sufficient and very modern stadiums to comfortably host the Asian Cup. The prime examples here are Xi'an, Dalian and Wuhan. Xi'an is well into the construction of a 60,000 seater, Dalian opened a 61,000 seater in 2013, and Wuhan opened a 30,000 seater this year. The one city I find a little amusing is Chengdu - they were renovating the city's main stadium for the 2021 Summer Universiade, and after ripping up the turf in the centre of the stadium they discovered ruins of a lost 1,000 year temple! So work has halted on that project, and a new football stadium in the north is now well under construction!
http://www.xinhuanet.com//english/2017-06/04/c_136338711.htm