The whole "Metro area is losing" population thing is interesting. When you say "the metro area has never lost population, until now" you are comparing decennial censuses (every 10 years) previously to this, which is a yearly estimation (and different from the ACS by the way). It's completely possible that sometime since 1950, for a year or two, the metro area lost population from one year to the next. Since that number was never estimated publicly though, you don't think the metro area has lost population. It's a completely incomplete statement not based off of complete evidence.
In any case, I saw an article today from Crain's in which a demographer states that the people leaving are most likely the ones who got screwed up by the recession but couldn't move away due to economic reasons (or something else). I would probably agree with that assessment considering both the city and area are basically at 10 year highs for employment.
Trying to draw a conclusion from one population statistic is extremely naive and shows a lack of understanding of any type of serious economics. You have to ask who is leaving, who is coming, why are the people coming and leaving, what is the jobs situation, what is the income situation, is the rate over time of certain groups of people leaving decreasing while others are increasing, etc. There are a lot of factors which you need to form an actual picture about the health of something.
For example, we can look at the labor situation (source: Bureau of Labor Statistics) versus when this estimate was baselining which is June 2016 and also June 2015:
Employment
* June 2006, Chicago MSA: 4,608,798
* June 2015, Chicago MSA: 4,660,559
* June 2016, Chicago MSA: 4,705,546
Difference (2015 to 2016): +44,987 (+0.97%)
* June 2006, Chicago city: 1,233,423
* June 2015, Chicago city: 1,288,942
* June 2016, Chicago city: 1,315,185
Difference (2015 to 2016): +26,243 (+2.04%)
Unemployment
* June 2006, Chicago MSA: 233,281
* June 2015, Chicago MSA: 300,192
* June 2016, Chicago MSA: 277,763
Difference (2015 to 2016): -22,429 (-7.47%)
* June 2006, Chicago city: 75,062
* June 2015, Chicago city: 95,292
* June 2016, Chicago city: 87,989
Difference (2015 to 2016): -7303 (-7.66%)
Labor Force
* June 2006, Chicago MSA: 4,841,731
* June 2015, Chicago MSA: 4,957,812
* June 2016, Chicago MSA: 5,054,240
Difference (2015 to 2016): +96,428 (+1.94%)
* June 2006, Chicago city: 1,307,358
* June 2015, Chicago city: 1,381,997
* June 2016, Chicago city: 1,411,048
Difference (2015 to 2016): +29,051 (+2.10%)
Unemployment Rate
* June 2006, Chicago MSA: 4.8%
* June 2015, Chicago MSA: 6.0%
* June 2016, Chicago MSA: 6.0%
Difference (2015 to 2016): Unchanged
* June 2006, Chicago city: 5.7%
* June 2015, Chicago city: 6.7%
* June 2016, Chicago city: 6.8%
Difference (2015 to 2016):Increase of 0.1% NOTE: Last official numbers (Nov 2016) show it as 6.1%
So what's going on here? Both the MSA and city increased the number of people in its labor force from 2015 to 2016, increased the amount of employed people from 2015 to 2016, and decreased the amount of unemployed people. Obviously there's something else going on there. While these numbers aren't eye popping, they are not a decline at all. They're still a growth in the labor market of the city and area. If you look at the population changes of just the city over the last few years, the areas losing population are the ones that are more blue collar which is why I tend to agree with the demographer of the article. The city as a whole is not necessarily in decline - overall it's actually the opposite but certain aspects of the city are in decline which are the reasons why we have areas like downtown, wicker park, logan square, etc which are growing pretty healthily or fast but the entire city looks more flat.
I think that the city's plan to invest $100M in areas that need it will be very important. If successful, it should turn around what looks like slow growth overall into a little bit faster growth overall and not just in a few areas. Hopefully it works. However, in my estimation we'll probably see pretty small changes one way or another but will hit an equilibrium point in regards to this and it'll eventually flip. Most of the people who want to leave have already left or are in the process of doing it. The amount of people in the declining areas is going to decrease year over year for awhile until this equilibrium point. The $100M plan should help with that overall and turn it more positive in these areas.