As a follow up to this answer to a M.Y. question, why is שליט"א more commonly used after a notable Rav's name than the term נירו יאיר (or נ"י, as an abbreviation)?
A wise person, such as a rav is called a תלמיד חכם meaning student" of a wise person (Thanks to @wfb for the clarity!) This implies that even though he is wise, he is still a student, because he always needs to continue learning.
Also Pirkei Avot 4:1 states:
בן זומא אומר:איזהו חכם? הלומד מכל אדם, שנאמר: (תהלים קיט צט): "מכל מלמדי השכלתי כי עדותיך שיחה לי
Ben Zoma says: Who is the wise one? He who learns from all men, as it says, "I have acquired understanding from all my teachers" (Psalms 119:99).
This confirms the same idea that people become wiser by continually learning from others.
The term נירו יאיר implies that the person should continue to shine his light by continually learning and teaching others, and becoming wiser. This seems. to me, to be a bigger bracha than שליט"א, which is just a bracha that the rav should live long, without directly indicating that he should become wiser and teach others.