"We will not convince secular Israelis that kollel students protect Israeli society no less than IDF soldiers."
Never mind secular Israelis - you won't convince anyone of that. On a theoretical level, it has a very shaky foundation. On a practical level, nobody really believes it - not even charedim.
Let's start with the theoretical level. To be sure, there are a small number of statements in Chazal which, at first glance, would seem to indicate that kollel students are as effective as soldiers. However, as with all statements of Chazal, these are tersely stated, open to a number of interpretations, and must be considered in light of various interpretations that have been presented by Rishonim and Acharonim - as well as in light of the factual reality.
The Gemara (Sotah 21a and Makkos 10a) says that the study of Torah protects a person from certain types of harm. Elsewhere, the Gemara (Bava Metzia 108a and Bava Basra 7b) rules that Torah scholars are exempt from the expense of building protective walls for the city, since they are protected by virtue of the Torah they learn. The Baal HaTurim (Devarim 1:3) says that a Torah scholar can protect forty thousand people around him.
But to which kind of Torah scholars does this apply? Maybe, for example, it only applies to those who are teaching others and are thus connected to them? And to which kinds of circumstances is it referring? Who says that it applies to kollel students of today, with regard to protection from the Arabs? And how exactly does it work - is it a linear correlation? When the country goes from twenty thousand people in kollel to forty thousand, by what percentage does the number of untimely deaths in Israel allegedly decrease? How many lives would be lost if twenty thousand people left kollel for a year to serve in the IDF?
Note that Responsa Radvaz 2:752 greatly restricts the extent of the Gemara's ruling about Torah scholars being exempt from contributing towards security, including stating that it does not apply in cases where the rabbis consider themselves in need of protection. (I have seen quotes of other sources that the exemption only applies to situations where the protection is from theft, and not when lives are in danger.)
Second, and most significantly: Regardless of the sources that someone might dig up/ reinterpret to claim that yeshivah and kollel students are protecting Israel, the bottom line is that (a) the facts on the ground demonstrate otherwise, and (b) when push comes to shove, the charedim don't even believe it themselves.
The facts on the ground - as the Gemara would say, הא קא חזינן דלאו הכי הוא! From the tragedy of the Holocaust, to the 1929 massacres in Chevron, to the murders several years ago at Mercaz HaRav Kook, it is evident that Torah students are not even automatically protected from harm themselves, let alone protecting others. And this is only military harm - there are plenty of other kinds of harm that affect Torah students, from illness to fires to road accidents. And Israel does not seem to be any safer now than in 1948, despite the fact that there are 40,000 extra people learning in yeshivah/ kollel.
The charedim don't even believe it themselves. In Kiryat Sefer and Betar, bastions of the charedi community which are full of kollelim, they have the same security fences and armed guards as every other town in Israel that is over the Green Line. They have the same protections against different types of harm; in fact, charedim often seek to get the best doctor, not just a regular doctor! Any charedi person, given the choice of living in a settlement with a kollel but no guards, or a settlement with guards but no kollel, would choose the latter.
Rav Shlomo Yosef Zevin, in a classic article on the drafting of yeshiva students (translated in Tradition, Fall 1985), put it best:
In 1929 at Hebron... didn't young students of the yeshiva, whose holiness shone like stars in the sky, fall before the malicious enemy? Please, did these martyrs need protection or not?... If you understand that the scholars don't need protection in relatively peaceful times and are exempt from building the protective walls, what consequence has this when compared to a life-and-death struggle, a war which is a mitzvah and in which all are obligated? The defense authorities ordered everyone to cover all windows as protection against shattering glass in case of an air raid. Would anyone think that some rabbis will not do so, claiming, "Rabbis do not need protection?" ...Why did rabbis leave areas under enemy fire along with the rest of the general population? Why did they not rely on this maxim?
So, if you want to claim that we need lots of people in kollel in order to rebuild Torah after the losses of the Holocaust (although there is vastly more Torah learned today than before the Holocaust), fine. But don't claim that you believe that kollel students are remotely equivalent to the IDF in terms of protecting the country. They're not, and you know it.