Y20 airframes produced today may very well be serving within PLAAF in 2055. 40 years really isn't that much for a large airlifter, especially one designed after 1960s. Actually, out of large airlifters (30 tons payload or more), only c141 lasted less - some 30 years. C5, Il76, An124 and An22 have all crossed the 40 year treshold. And c17 is virtually guaranteed to cross it as well.
Even c141 might have lasted longer if it wasn't designed as a very narrow plane, for a less broad list of missions. They tried to remedy it with remanufacturing them as longer planes to get more cargo area but in the end it wasn't worth it.
We will probably see a smaller cargo plane in the next decade or so, we might even see a larger cargo plane some day - serving alongside y20, but i'd be VERY surprised if there's a same class replacement for y20 for the next 35-40 years.