According to Haomi Hanaoka McGloin
んです・のです functions to mark information as known in the context of the discourse.
のです allows the speaker to present information as if it were shared information. Depending on the context and type of sentence, the specific meaning varies however. There seem to be at least five different ways it can be used. In this instance I think the sentence is stated as an explanation.
Explanation type of usage:
Please excuse us. However (it is that) we are in a hurry as well. (T)
Wouldn't you like some apple pie? (It is that) I baked it for you. (T)
(It is that) I'm just waiting for a friend. (T)
Especially if you include the sentences that precede the one of the question (which comes from http://www.alc.co.jp/), it seems to fit the pattern:
So, I'd like to explain the background behind the epidemic of "stylish high school students" of these last few years. First of all, I think (it is that) Japan has started to change. (It is that) there are definitely fewer and fewer Japanese who are afraid to stand out and must do things together.
Other forms of usage that Hanaoka McGloin mentions are the following.
Conjecture: used in questions or with
でしょう. It is used when there is reason to assume that something is the case:
(Is it that) we have run out of toner? (T)
Hanaoka McGloin gives:
Is it that it is raining? (someone thinks it might be raining).
And contrasts it with:
Is it raining? (neutral question)
And she warns that in neutral information questions with no hint as to what is the case,
ん・のです is wrong:
can not be asked as
If there is no explanation, or conjecture, it might be rapport. It might resemble English "you know" or show an emotional involvement:
I'm on the phone! (T)
In sentences with から it can have a reproachful tone:
Since he is new, do go easy on him. (T)
んですけれど it can be "back-grounding" of information, introducing useful information for what follows:
At first we used to go separately, but one day we started going and returning together. (T)
Finally Hanaoka McGloin warns that
んです・のです can be offensive, because of the possible suggestion that the other should have known, as in point 4 for example, or in Hanaoka McGloin's example:
A: "Shall I do it?"
B: "No, I am going to do it" [and you should have known that]
Most examples (T) from http://tatoeba.org/. Explanation adapted from Naomi Hanaoka McCloin's /A students' guide to Japanese Grammar/ (Taishukan Publishing Company, 1989).
確実にin this context is ambiguous as to what it is modifying. In one interpretation, as in your translation, it modifies the writer's judgement ('definitely'). In another interpretation, it modifies the manner of decrease ('steadily/monotonic decrease').
のだputs emphasis on the statement, Most often (but not always) it implies that it is an explanation to what was said in the preceding context. A good translation is
It is the case that .... You definitely need to choose the right informant. Just being a native does not ensure they can give you the right information about Japanese.