In the 5 pillars which is the only requirement to call oneself a muslim, is to believe in Mohd as a prophet and NOT the finality.
So it is legit for ahmadis to call themselves muslims if they believe in 5 pillars.
in Quran Prophet Muhammad is declard "Seal of Prophets" Khatim un Nabiyyin, and just like a seal is placed at the end of a document and to finish something so was Prophet Muhammad sent as a final prophet. And we take into the Ahadith as well not just Quran: "My position in relation to the prophets who came before me can be explained by the following example: A man erected a building and adorned this edifice with great beauty, but he left an empty niche, in the corner where just one brick was missing. People looked around the building and marveled at its beauty, but wondered why a brick was missing from that niche? I am like unto that one missing brick and I am the last in the line of the Prophets."
(Bukhari, Muslim,Tirmidhi, Musnad Ahmad, Tirmizi, Babu Khatimin-Nabiyyin, Musnad Abu Dawud Tayalisi)
"I am Muhammad, I am Ahmad, I am the effacer and infidelity shall be erased through me; I am the assembler. People shall be assembled on Doomsday after my time. And I am the last in the sense that no prophet shall succeed me."
(Bukhari, Muslim, Tirmidhi, Muatta')
and now the Ayats of Quran: "Muhammad is not the father of any of your men, but (he is) the Messenger of Allah, and the Seal of the Prophets: and Allah has full knowledge of all things."
(The Holy Quran, Al-Ahzab 33:40)
"This day have I perfected your religion for you, completed my favor upon you and have chosen for you Islam as your religion."
(The Holy Quran, Al-Maidah 5:3)
so no more addition in religion is acceptable by any mirza ghulam qadiyani or any other person
"O ye who believe! Obey Allah, and obey the Messenger, and those charged with authority among you.
If ye differ in anything among yourselves, refer it to Allah and His Messenger, if ye do believe in Allah and the Last Day: That is best, and most suitable for final determination."
(The Holy Quran, An-Nisa, 4:59)
the bold is to establish the authenticity of following of Ahadith of Prophet Muhammad