Neither rasool Allah pbuh, nor the companions, nooooor the predecessors congratulated for Mawlid ! Bidaaaaah bro, bidaah
What you are saying is based on the idea (mostly propounded by Wahhabis) that any type of worship or Islamic celebration that was not performed by the early Muslims is automatically an evil and forbidden thing. People who disagree with the Wahhabis and believe that celebrating the Mawlid is acceptable include: Yusuf al-Qaradawi (al-Azhar scholar), Imam al-Nawawi, Ibn Hajar al-Asqalani, al-Suyuti, Muhammad al-Hadhrami (Shafi`i jurist), Sadr al-Din al-Jazari (Shafi`i jurist).
I don’t celebrate the Prophet’s birthday myself ﷺ, but since some people enjoy it and get something out of it, I have no problem with them doing it. They have a desire to feel close to the Prophet and the mawlid celebrations fulfill that desire for them, and as the above list of scholars should show, there is no consensus on forbidding such celebrations. You are free to not celebrate it yourself, but you have no right to ruin it for others. If someone says mawlid mubarak, it is politeness to reply to them in the same kind.