It actually makes some sense not to give new planes to units currently operating Q5. Surely part of the reason is money, perhaps even that's the biggest part.
But there's also the role of the plane and the training needed to switch from it to something newer.
Q5 pilots trained exclusively for low level subsonic flight, for short range air support missions, possibly with fairly little night training. and most of their trained attacks were for very close range unguided weapon attacks. (more units had no models that could use targeting pods than units that did have them. And it's also unlikely there were targeting pods available for most planes per unit)
Ground crews were working on basically the same plane for their entire career. Plane that is technologically from 1950s, with perhaps some newer subsystems.
Airbase infrastructure was dedicated to such an old plane....
And now to jump from Q5 to J16, which would have a multirole mission, and its air to ground capabilities would require a much broader training course ... it just doesn't make much sense IF PLAAF is looking where to save money.
Of course, a more logical course of events would be that J16 goes to JH7 units and those JH7 planes get handed down to Q5 units. But it'd still be quite a jump, both in infrastructure, and mission profile so considerate retraining would need to be done.
And for either option one would have to have more people per same number of planes in an unit than with Q5. both jh7 and j16 are likely to require more groundcrew and they require double the number of pilots.
So while none of the above is necessarily a showstopper, if PLAAF wanted simply the best and money was not an issue - eventually all retraining and infrastructure re-work could be performed, IF PLAAF is indeed looking to maximize its invested money, outright abolishing Q5 units does seem like a fairly prudent way to go.