The PLA already had nearly 30 years to make the shift, starting from the PLL-01 of which was first noted back in the 1990s. And with the downsizing of ground forces, the amount of towed artillery needed would decrease as well. If the PLA had any intention of getting a towed 155mm howitzer, we should have at least seen a couple of examples by now. To be fair this does not preclude the PLA from making the shift later in the future, but suffice to say that it is not on their minds as of now. And I don't see how towed artillery being placed on the back seat means spending more money on them is worthwhile seeing as they will be replaced eventually, then what is China going to do with all those modern shells ? These are even less likely to be fired than general ammunition, especially with the prospect of China getting into a serious land war requiring huge numbers of towed artillery being nill in the forseeable future.I'm not sure if it's out of a "desire" to keep both in existence, rather the cost of replacing all of their older 152mm guns with new 155mms in a fashion we would consider quick.
For all we know it might end up taking them some 30 years to fully switch from 152mm to 155mm from when the first PLZ05 was introduced, in which case it would definitely make sense to allow the remaining 152mm guns to be capable of firing somewhat more modern shells.
The decision to procure new artillery in 155mm rather than 152mm would have been a conscious decision, not something made on a whim. I.e.: I think it is reasonable to assume some kind of benefit or preferable capability could be derived from moving to 155mm vs 152mm.
The question we need to ask instead then becomes one of "why is the shift from 152mm to fully being 155mm taking relatively long". And for that, I think the answer becomes a combination of cost and priority in terms of overall PLA funding.
And I agree that military decisions are not made on whim, but it does not preclude existing biases and/or omissions and oversight to influence their judgement either. For all we know it is just as likely the newer corps of officers had replaced the old ones without consulting the previous decisions making reasons.