I find the above question and answer very interesting.
I've always thought that given the sheer number of VFs in an average colony fleet (thousands of VFs), changing to a new model must be quite a long, slow process. Not only do you actually have manufacturer the new VFs, but you also need to do conversion training for all your pilots.
Likely, new VFs are introduced as attrition replacements, except in time of war against a superior enemy, when production of the new model is likely fast-tracked.
Kawamori's answer is telling, in that I read ithe "probably some fleets out there that are still using the VF-19s" as implying that the VF-19 is considered quite an old model (as it should be as it's a near 20 year old design) and only a few "some" fleets are still using it.
Graham