In fact, it can be argued from the holy Qur'an that celebrating mawlud should be deemed an act that is at least permitted, mubah.

This is because if you carry out an act that expresses love or shows respect (or compassion) toward something that is regarded as something from among the symbols of Allah, then this show of love, respect, compassion etc is an indication of deep level of piety in the heart of such a person.

Qur'an 22:32
...if one observes the sanctity of the symbols of Allah, then such things emanate from the piety of the hearts.

...It means that respect for the Symbols of Allah is an evidence of the piety of heart, which indicates that the symbols of Allah are respected by the one who has taqwa (fear of Allah) in his heart and that taqwa is a quality which rests in one's heart...
Tafsir (Ma'ariful Qur'an)

Also it can be submitted that among the sha'airAllah, the coming of the prophet, sallAllahu alayhi wa sallam, is the most prominent. It is the moment the heart that is going to carry the message of Islam was delivered to the world. All other sha'airAllah became known to us through him, sallAllahu alayhi wa sallam.

It is mentioned in the Qur'an:
O Prophet, indeed We have sent you as a witness and a bringer of good tidings and a warner.
33:45

He, sallAllahu alayhi wa sallam, said in a hadith:
The Hour and I have been sent as close together as these two (his two fingers, sallAllahu alayhi wa sallam).
Sunan An-Nasa’i 1578

Now, this very important landmark for the entire humanity comes around only once in a year and some people are saying that it should not elicit any kind of reaction whatsoever from the entire muslim ummah? We should just keep mum like not a fly passed by?

For those who say the sahaba i kiram, radiyAllahu anh, did not celebrate mawlud, they should recall that the sahaba i kiram, radiyAllahu anhum, on a daily basis, showed personal connections to the prophet, sallAllahu alayhi wa sallam, out of respect and honor and that is besides what the religion has obligated upon them. They would kiss his hand, catch his spit etc. Were those actions recommended acts of worship? Of course, no. They did it out of love, respect and honor for him, sallAllahu alayhi wa sallam. However, us later muslims do not have this opportunity to connect to the personality of the prophet, sallAllahu alayhi wa sallam, like the sahaba, radiyAllahu anhum, had. Mawlud nabiyy offers that opportunity my dear brothers.

Imam Malik, rahmatUllahi alayh, is said to have rejected maulud nabiyy as quoted by @lexiconkabir.
Imaam Maalik said:
"The last generations of this ummah can only be rectified by that which rectified its first generations. What was not part of the religion then cannot become part of the religion now"

We ask: Where do they get this statement from if maulud nabiyy is meant to have been a sixth or seventh century innovation?