(כה) וַיֹּ֨אמֶר יהוה אֶל־מֹשֶׁ֗ה הָשֵׁ֞ב אֶת־מַטֵּ֤ה אַהֲרֹן֙ לִפְנֵ֣י הָעֵד֔וּת לְמִשְׁמֶ֥רֶת לְא֖וֹת לִבְנֵי־מֶ֑רִי וּתְכַ֧ל תְּלוּנֹּתָ֛ם מֵעָלַ֖י וְלֹ֥א יָמֻֽתוּ׃ (כו) וַיַּ֖עַשׂ מֹשֶׁ֑ה כַּאֲשֶׁ֨ר צִוָּ֧ה יהוה אֹת֖וֹ כֵּ֥ן עָשָֽׂה׃ {פ} (כז) וַיֹּֽאמְרוּ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל אֶל־מֹשֶׁ֖ה לֵאמֹ֑ר הֵ֥ן גָּוַ֛עְנוּ אָבַ֖דְנוּ כֻּלָּ֥נוּ אָבָֽדְנוּ׃ (כח) כֹּ֣ל הַקָּרֵ֧ב ׀ הַקָּרֵ֛ב אֶל־מִשְׁכַּ֥ן יהוה יָמ֑וּת הַאִ֥ם תַּ֖מְנוּ לִגְוֺֽעַ׃ {ס}
(25) יהוה said to Moses, “Put Aaron’s staff back before the Pact, to be kept as a lesson to rebels, so that their mutterings against Me may cease, lest they die.” (26) This Moses did; just as יהוה had commanded him, so he did. (27) But the Israelites said to Moses, “Lo, we perish! We are lost, all of us lost! (28) Everyone who so much as ventures near יהוה’s Tabernacle must die. Alas, we are doomed to perish!”
Read on another level, the rebellion narratives are largely concerned with the issue of human leadership, its qualifications, manifestations, and limitations (and even failures). - Everett Fox
(א) למשמרת לאות. לְזִכָּרוֹן שֶׁבָּחַרְתִּי בְּאַהֲרֹן לְכֹהֵן וְלֹא יִלּוֹנוּ עוֹד עַל הַכְּהֻנָּה:
(1) למשמרת לאות TO BE KEPT AS A SIGN — i.e. as a reminder that I have chosen Aaron to be the priest and that they should therefore not murmur any more against the priesthood.
(ג) וַיִּֽקָּהֲל֞וּ עַל־מֹשֶׁ֣ה וְעַֽל־אַהֲרֹ֗ן וַיֹּאמְר֣וּ אֲלֵהֶם֮ רַב־לָכֶם֒ כִּ֤י כׇל־הָֽעֵדָה֙ כֻּלָּ֣ם קְדֹשִׁ֔ים וּבְתוֹכָ֖ם יהוה וּמַדּ֥וּעַ תִּֽתְנַשְּׂא֖וּ עַל־קְהַ֥ל יהוה׃
(3) They combined against Moses and Aaron and said to them, “You have gone too far! For all the community are holy, all of them, and יהוה is in their midst. Why then do you raise yourselves above יהוה’s congregation?”
What's the reason the people should not 'murmur against the priesthood"?
A different view on the meaning of Aaron's staff...
But this rod was only a sign that the tribe of Levi was chosen from the rest of the tribes, but did not [serve as a sign] that Aaron should have the priesthood [since the people did not dispute his right to it ever since the Heavenly fire had descended on the eighth day of the installation of the priests and consumed Aaron’s offerings, as explained above]. The correct interpretation is that [the expression] to be kept there, for a token means [that the budding of the rod of the house of Levi was to serve as a reminder to Israel that G-d had chosen] the tribe of Levi in exchange for the firstborn. This is because the burning [of Aaron’s offerings by the Heavenly fire] was a sign [that Aaron had been chosen] for the priesthood, and the budding [of the rod was an indication] that the Levites [had been selected for their functions in the Tabernacle], as I have explained. - Ramban
"That their grumblings may be finished...
in the future such as when king Uziah arrogated to himself to offer incense in the Temple (Chronicles Ii 26,15). The construction of the word ותכל when we would have expected ותכלה, is parallel to the occasions when the Torah writes צו! as an abbreviated imperative instead of צוה, “tzaveh!” - Rashbam
(טז) וּכְחֶזְקָת֗וֹ גָּבַ֤הּ לִבּוֹ֙ עַד־לְהַשְׁחִ֔ית וַיִּמְעַ֖ל בַּיהוה אֱלֹהָ֑יו וַיָּבֹא֙ אֶל־הֵיכַ֣ל יהוה לְהַקְטִ֖יר עַל־מִזְבַּ֥ח הַקְּטֹֽרֶת׃
(16) When he was strong, he grew so arrogant he acted corruptly: he trespassed against his God by entering the Temple of the LORD to offer incense on the incense altar.
To clarify the disagreement between Rashi and Ramban
“as a safekeeping to serve as a sign for the rebellious ones.” Rashi understands this line as “a reminder that I have selected Aaron as My Priest, and that further complaints about the priesthood should cease.” Nachmanides counters that the staff and its blossoming was a sign only that the tribe of Levi enjoys a special status among the other tribes, but it did not signify anything about the priesthood. He therefore understands the word לאות as a reference to the special status of the tribe of Levi. He claims that there was no need at all to furnish a sign that the priesthood belonged to Aaron, seeing that whenever heavenly fire descended on the censer of Aaron and he did not get burned by it this was sufficient proof that G’d approved of him in his capacity as High Priest. This is also the meaning of the words והנה פרח מטה אהרן לבית לוי, “here the staff of Aaron, member of the house of Levi blossomed.” Aaron’s position among the tribe of Levi, which was already special, flowered even further. (Tur)
(כז) וַיֹּֽאמְרוּ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל אֶל־מֹשֶׁ֖ה לֵאמֹ֑ר הֵ֥ן גָּוַ֛עְנוּ אָבַ֖דְנוּ כֻּלָּ֥נוּ אָבָֽדְנוּ׃
(27) But the Israelites said to Moses, “Lo, we perish! We are lost, all of us lost!
(א) ויאמרו בני ישראל. די לנו שגוענו ואבדנו והנה עוד כל הקרב אל המשכן יהוה ימות כמות השרופים ומי יתן ותמנו לגוע אז אמר השם לאהרן שהוא הנגיד על שבט לוי להזהירו שלא יכשילו הלוים את ישראל. וטעם לאהרן על יד משה או כאשר הוא כי גם אהרן נביא היה גם זאת הפרשה לאות כי במדבר סיני היתה כאשר הוקם המשכן ונבחרו הלוים ונפסלו הבכורים:
(1) AND THE CHILDREN OF ISRAEL SPOKE UNTO MOSES, SAYING. It is enough for us that we perished and were undone. Now, in addition, everyone that comes near the tabernacle will die like those who were burnt. O that we should cease dying. God then spoke to Aaron, who was the leader of the tribe of Levi, and cautioned him that the Levites not cause the Israelites to stumble. The meaning of unto Aaron (Num. 18:1) is that God communicated with Aaron through Moses. On the other hand, the clause may be taken at face value, because Aaron too was a prophet. Also, this section is proof that the rebellion took place in the desert of Sinai when the tabernacle was erected and the Levites were chosen, and the first-born were disqualified.
The question: “will we ever stop perishing?” (verse 28) suggested that the Israelites viewed the Levites as the cause of their deaths. This is why G’d told Aaron in his capacity as the tribal head of the tribe of Levi to take care that the Levites should not become the cause of Israelites dying through any error they would commit (compare Ibn Ezra on verse 27). - Rabbenu Bahya
(טו) וּשְׂעִ֨יר עִזִּ֥ים אֶחָ֛ד לְחַטָּ֖את לַיהוה עַל־עֹלַ֧ת הַתָּמִ֛יד יֵעָשֶׂ֖ה וְנִסְכּֽוֹ׃ {ס} (טז) וּבַחֹ֣דֶשׁ הָרִאשׁ֗וֹן בְּאַרְבָּעָ֥ה עָשָׂ֛ר י֖וֹם לַחֹ֑דֶשׁ פֶּ֖סַח לַיהוה׃
https://youtu.be/Rwd08M7pr3A?si=77qOBg_3nsP9Gdo7