Quran 17:59
So that they have no excuses.
I think the primary purpose of miracles would be to prove that a prophet of God is from God.
Let me try to put a wrapper around this conversation. If Allah wanted to make a believer of everyone in the World he could have done it in a snap or Allah would have come down and shown himself to everyone ( From Quran). But we must understand that this is not the design. When we are born, he puts it in our subconscious that there is one creator of universe (From Quran). It is our test to accept or reject this guidance. Allah’s responsibility after we are born is perhaps only to have the message delivered to us that contains clues that match with our subconscious memory. But, because if our vanity or other Wordly reasons, people ignore and reject Allah even when their subconscious is screaming otherwise. Eventually they make that voice permanently quiet or if they conyinue on the evil ways, the voice of evil takes over. Allah just gives us with clues for the sincere ones to match that internal guidance. The same clues either in the universe or in Quran are taken two ways. For sincere ones it guides them to Allah and they infer positive meaning and for those with wrong intention the same clues help them turn away from Allah. Both see the same information, but they take a different meaning. If anyone is expecting an absolute proof of Allah before accepting him, he/she will never find it by arguments, facts or figures, but only in his/her heart. That is the design based on my understanding of Quran.
To my understsding, Faisal agrees. This answers your question as miracles were needed in the past for couple of reasons you stated. Now people are smarter, with all kinds of info available on their fingertip and science is advanced enough and we have among us the biggest miracle of all the Quran with all signs and clues for the sincere ones. So no need of miracles any more.