The simplest answer to the latter question is as follows: I don't know. At least, I can't be certain. I was vegetarian for twelve years (I made the switch at age six) and have been vegan for almost three, so I can't speak from personal experience - I've never gone back to eating meat. I can, however, offer a hypothesis for those who may be interested.

To answer this question, I'll begin by offering you the simplest, most accurate description of veganism I've found to date. This one comes from The Vegan Society:

"Veganism is the practice of abstaining from the use of animal products, particularly in diet, as well as following an associated philosophy that rejects the commodity status of sentient animals."

The second part of this description is just as important as the first. Vegans reject the commodity status of sentient animals. Consequently, we don't use animal products, since doing so would promote the commodification of sentient beings. So, in reality, veganism is much more than a diet. It's not simply plant-based eating, it's a moral and ethical philosophy (or, as some call it, a lifestyle). 

People who eat plant-based diets for the right reasons (i.e. to prevent animal suffering or to help the environment rather than purely for their own health) very rarely return to eating animal products. A strongly ingrained moral philosophy is difficult to rid of, and the majority of people would see no reason to rid of their ethical beliefs in the first place.

"Veganism isn't just a phase. It's a lifelong moral commitment."

Let's say you're opposed to murder (as most people are), and have felt this way for your whole life. You hold a strong and deeply ingrained moral belief that it's completely unacceptable to maliciously take another person's life. It'd be very unlikely for you to suddenly pull a one-eighty and decide that murder is A-okay, right?  The same goes for vegans. We strongly oppose the abuse and commodification of animals. Most of us have felt this way our whole lives - for some, it just takes a while to make the connection between animal products and animal suffering. Once we make the connection, however, we're not going to go back. The only way you could go back would be if you gave up your moral beliefs - something that, as I mentioned, rarely happens. That's why veganism isn't just a phase. It's a lifelong moral commitment.

But what about vegetarians?

Well - that's a little different. A large proportion of vegetarians do avoid meat for ethical reasons, but others only do it for their health... and they fit the definition of vegetarianism just as well as the ethical vegheads. There are several reasons why health-based vegetarians could go back to eating meat. Perhaps their vegetarianism was only a trial or a short 'health kick'. Perhaps they didn't want to deal with criticism from family members, friends and strangers alike. Perhaps they didn't give their bodies enough time to adjust to the new diet (so they felt sick and thought they needed meat), or perhaps they just sucked hard at making vegetarian food and went back to what they were used to. Ethical vegetarians, on the other hand, usually stick to their guns or move on to veganism. I'm an example of one such individual - as mentioned, I was vegetarian for twelve years before becoming vegan.

I haven't eaten meat in over fourteen years now, and I made the initial decision when I was six years old. So, at least in my case, I can be certain that my abstinence from the use of animal products is not a phase. There are many others in the same boat as me - if you take a look through the comments of the original article, many people have been vegan or vegetarian for ten, twenty, thirty, forty years.

I haven't yet mentioned the sample size of the survey 'proving' that vegetarianism/veganism is just a phase - 11,000. That's quite a meager sum when considering the existence of the 16 million vegetarians and vegans in the USA alone. Similarly, the survey was confined solely to the United States, a country where meat consumption levels are massive, advertising of animal products is rampant, and various companies push the notion that animal products are necessary for good health (which is of course untrue). The 399+ million vegetarians (40% of the population) in India, for example, were ignored.

"There are enough vegetarians and vegans out there who are dedicated to their morals and would never give up on their beliefs to prove that it's the furthest thing from 'just a phase'."

Based on everything I've just mentioned, I think it's fair to conclude that the aforementioned article is far from the truth. It's a perfect example of what some would call "anti-veg propaganda". There are enough vegetarians and vegans out there who are dedicated to their morals and would never give up on their beliefs to prove that it's the furthest thing from "just a phase". Meat, cheese and eggs don't even look or smell like food anymore. I can't look at a hot chicken at the supermarket without feeling sick and mournful. It's more than just about the food, though - veganism is an expression of my soul. I, for one, know damn well that's never going to change.