I am thinking that it is just an idea the airline has been asked for many times - the implementing of it will in part be determined by the response from the general public. They would have to be the smallest cats and dogs on the planet to be able to fit in any carrier that would actually fit under the seat in front. Allergies are one consideration, but so are issues surrounding genuine service dogs and any issues that may present when "fluffy" just magically escapes its crate, how domestic pets will actually "behave" at 30,000 feet - what if another passenger gets bitten or scratched, any passenger that gets assigned a seat alongside a passenger with a fur companion and doesn't wish to consume their meal with animals present, what the reaction will be from Biosecurity Detector dogs that work in the terminal space, whether these pets must remain in their crates until they are completely through the airport arrival process, what happens to escaping pets in pick-up areas or terminal car parks. Who will actually "police" the whole issue to ensure passengers travelling with onboard pets remain compliant and keep them in the crate? There are hundreds of issues to consider - sounds like a fun idea to begin with but the logistics of it are a nightmare. And in the vast majority of cases it will not be the animal that is the issue, it will be the owner. Pets travel extraordinarily well in the cargo hold - probably best to just leave them where they are safe and secure.