Bulgarian here, there are a lot of aspects the article does not cover.

First one is that up until 1989, Bulgaria was largely an agriculture state with the soviet union being the only market. A heavily controlled market at that. Some might argue that Bulgaria used to produce computers but it was mostly a small assembly line using Intel chips, so no, not really. Mind you the math education was on a good level, hence the reason why so many people can easily jump into cs. But families like most rural countries tended to be large. I happen to have a family tree dating back to the late 17-th century and most of the people back then had 8+ children. Needless to say, that isn't the case anymore. Looking at people around me, I can only think of one or two examples that have more than 1 child. Combine that with the other factors in the articles and you start seeing the bigger picture.

Now, on immigration: I was a child in the early 90's when the first real information from the west started coming in. And as people tend to do, we only saw what we wanted to see. I grew up daydreaming about the day I'd move west where everything is bread and butter. And I did as early as I could: I was ~16 at the time. I was pretty disappointed in a lot of aspects: life abroad was not as advertised. It turned out to be complicated and rough. Yes, roads were better, streats were cleaner and people had a tendency to try to maintain the environment around them. Something which has started to happen here noticably. But there I was faced with entirely new problems which I would have never imagined could exist in the real world.

As I went through university, some other rough moments hit me hard and at the end I was so fed up, I wanted to move away from there. As I was talking to old friends, I figured that as a software engineer with a university degree, moving back to Bulgaria might be a possible option. Back then getting a work permit was still difficult as a new EU member state so I decided to skip the hassle and move back and give it a shot. Bottom line is it turned out to be a a great solution.

Like most market economies these days, young people and by extension working people gravitate towards the big cities. And naturally the smaller cities are dying out.

Because life is incredibly cheap compared to most of Europe, your odds of making a decent life are much greater - the bar can be set much lower. To give you an idea - my monthly paycheck at my first job was just over 700 euros(underpaid even for a developer with little to no experience). I had rent, bills, transportation and food to take care of and even with that, I was still able to save up around 250/month. Something unthinkable for most European capitals on such a tight budget. Now, many years later, things look really well for me.

Ocassionally you do hear someone on the streets saying something along the lines of "See this points at something, this would never happen in any other country". Since what I've said here has been a subject of discussion with many friends, we usually look at each other, roll our eyes and walk away. Which brings me to the final point:

The article doesn't cover the large amount of people returning home after spending years abroad either. And looking at many of my friends, the number is pretty high: a good 60-70% at a glance. Far not only software engineers(who on average make a few times the national average).

What I'm trying to say is that a lot of people still live with the illusion that in countries such as France, Germany, Italy and so on, everyone has a 1000 square meter mansion with 2 hellicopters and a collection of one-off hypercars. Which is not to say that my life abroad was a bad experience - on the contrary. If anything, it made me a lot more resiliant and capable of handling difficult situations. As a matter of fact I'd encourage young people to pack a bag and move abroad for a year while you are still 19(referring to the intro of the first book of the Dark Tower series titled "What it's like to be 19").