How is the “best [heart/lung/ENT/etc.] doctor” determined? How do we really know? Answer: we — the laymen — don’t. We are not in a position to know. All the heart experts in a given location, between themselves, know who is the biggest expert among them. What the first-year med students say is immaterial; what the taxi drivers in town say is immaterial; what the hardware store owners say is also immaterial. What the other experts say is what counts. The experts know who among them is the biggest expert.
It is similar when trying to quantify gadlus in Torah. The opinion of those not in a position to know simply does not count. The top Torah scholars in every generation all know who among them is the greatest Talmid Chacham, who is understood to be the greatest sage (“gadol”) of the generation. Occasionally there might be more than one gadol, but the method of determining that scholar’s gadlus is the same.
For the record: a psak halacha forbidding ascending the Har HaBayis was issued several times over the past hundred years, signed by all the greatest tzadikkim, rabbonim, poskim of the time. The first one that I know about was issued in the 1920s, and included the signature of the senior Rav Kook. After the Six Day War, the gedolim of the time also publicized their opinion forbidding going onto the Har Habayis. More recently, Rav Elyashiv zt”l constantly publicized his opinion forbidding ascending the Har Habayis.