I wish it was easy as that @Zetirian, but it isn't.
From the few statements Tess has made on things concerning rarity, it's pretty clear that older pets are indeed intended to be rarer than newer pets on average and that every pet has an intended rarity that they try to uphold through the re-release.
Plus, there's a math problem with your example of 5 VRs = 1 New Rarity (going by your numbers): What about those VRs that have an actual ratio of 1:200? Would you still expect people to pay 5 of them? How do you want to tell if a pet is rarer than another one even with a new rarity tag? This is the exact reason why we use age, because it's the safest bet. Nothing will ever change that.
Apart from that, as I've said before - I'm pretty sure the number of active players is somewhat stable. If not, they would hire more staff and not simply re-stock the numbers when staff members have quit. Of course there are more active players during december, but apart from that time of the year, the quitting and joining members even each other out.