The number have clearly decreases remains 1300 + 1100 B/D with 105 mm on 7500 tanks.
Why? modernised T-59 is still good enough against 80% of the potential enemies
You always have limited resources, so use it wisely is the way to go ... you don't need a howitzer to kill a rabbit
I am not familiar with maintenance costs for the Type, but from the point of view of replacing parts-there are a great many of the same/similar model being decommissioned every year. These would be used for scrap if the entire fleet was decommissioned. Unfortunately, there is also the issue of a great deal of munitions for the 100mm cannon sitting around-a problem the Russian Federation had too following the breakup for the SU, albeit on a much larger scale. Thankfully, the PLA did not make the mistake of trying to mass-adapt IFVs with 100mm cannons, as BMP service history in Chechnya/ M1128 in Afghanistan etc. have proven this to be a terrible idea.
At the end of the day, one can never be too sure that these vehicles will never be needed. There would be a large gap in the PLAGF's tank arsenal to replace, and as others have pointed out here; a tank is still a tank. It still outclasses contemporary IFVs like the Stryker and Bradley for comparison.