The two main arguments for traditional attribution and early dating is that
A) 4 church fathers attribute the gospels to the traditional names
B) The gospels do not mention the destruction of the temple.
Response to A)
The problem with this argument is that Mark, Matthew and Luke do not identify the author in anyway nor do they tell us their credentials.
The problem is that any text with an unknown author is not particularly reliable as we can all tell from chain letters, anonymous online articles, etc.
As Matthew Ferguson notes
Thucydides’ History of the Peloponnesian War (1:1), which states at the beginning: “Thucydides, an Athenian, wrote the history of the war between the Peloponnesians and the Athenians, as they fought against each other.” The historians Herodotus (1:1), Dionysius of Halicarnassus (1.8.4), and Josephus (BJ 1.3) all likewise include their names in prologues. Sometimes an author’s name can also appear later in the text. In his Life of Otho (10.1), for example, the biographer Suetonius Tranquillus refers to “my father, Suetonius Laetus,” which thus identifies his own family name.
Here is one example
Joseph, the son of Matthias, by birth a Hebrew, a priest also, and one who at first fought against the Romans myself, and was forced to be present at what was done afterwards, [am the author of this work].
Josephus, Jewish war 1.3
So We already have evidence against the traditional attribution.
The argument for traditional attribution is that 4 Christians namely Papias, Irenaeus, Tertullian and Clement of Alexandria make that claim as well.
Firstly, we do not start getting a consensus by Irenaeus, Tertullian and Clement of Alexandria until the third century about first century documents which is needless to say beyond useless.
So Let's take about Papias because really it is the only thing that traditional attribution has.
1) Papias writings and claims are all lost. The book where Papias tells us the attribution is called Exposition of the oracles of the Lord and it is lost, we have no manuscripts of it. That passage about attribution is only quoted in the fourth century by a Christian named Eusebius. So in reality, we do not have any early second century evidence since the writings from that time period by Papias were lost but we what we have is fourth century evidence by Eusebius.
2) Papias makes no claims about the authorship of Luke and John. Only Mark and Matthew.
3) Papias was not talking about our canonical gospels. Here is the passage from Eusebius
Let's look at Matthew for example.
Therefore Matthew put the logia in an ordered arrangement in the Hebrew language, but each person interpreted them as best he could.
Eusebius, "History of the Church" 3.39.14-17,
First of all, our Matthew is not a logia. It is a narrative that goes chronologically from nativity to virign birth to ministry to passion, crucifixion and resurrection. It includes not only the sayings of jesus but also his deeds, actions, miracles, what others say about him, the words of his disciples, what Jews said about him (The Guard's report) and etc. And the scholarly consensus is that gMatthew is a Greek document based on a greek document not written in Hebrew.
Not to mention that logion has high connotation as specifically meaning a saying of Jesus that is NOT found in our canonical gospels.
4) Papias was unreliable according to the only source that quotes him.
According to Eusebius, the only man who ever quotes Papias, Papias is a man of "very little intelligence" (Ecclesiastical history of the Church 3.39.13)
5) Papias was gullible
There is a report according to a scholium attributed to Apollinaris of Laodicea, that says
Judas did not die by hanging[50] but lived on, having been cut down before he choked to death. Indeed, the Acts of the Apostles makes this clear: Falling headlong he burst open in the middle and his intestines spilled out.[51] Papias, the disciple of John, recounts this more clearly in the fourth book of the Exposition of the Sayings of the Lord, as follows:
Judas was a terrible, walking example of ungodliness in this world, his flesh so bloated that he was not able to pass through a place where a wagon passes easily, not even his bloated head by itself. For his eyelids, they say, were so swollen that he could not see the light at all, and his eyes could not be seen, even by a doctor using an optical instrument, so far had they sunk below the outer surface. His genitals appeared more loathsome and larger than anyone else's, and when he relieved himself there passed through it pus and worms from every part of his body, much to his shame. After much agony and punishment, they say, he finally died in his own place, and because of the stench the area is deserted and uninhabitable even now; in fact, to this day one cannot pass that place without holding one's nose, so great was the discharge from his body, and so far did it spread over the ground.
b) Early dating: The gospels do not mention the destruction of the temple
1) Argument from Silence.
Imagine if Historians 1000 years from now could recover nothing from our period but the state of the union address of Trump. The 2018 state of the union address does not mention 9/11 attacks. Does that mean that 9/11 happened after 2018. Not really. A really good explanation is that the document was written after the destruction of the temple that everybody already knows about it and it would be frivolous to mention something that your audience knows about just like Trump does not talk about 9/11 because everyone knows about it.
2) A lot of extrabiblical writings like the Gospel of Barnabas, Second Treatise of the Great Seth (which has first-hand eyewitness testimony from jesus himself), the second apocalypse of Peter, Gospel of Judas, etc. also do not mention the destruction of the temple yet I doubt Christians would want to date it before 70 AD.
I think these are all the arguments proposed to defend that the gospels were written by eyewitnesses before 70 AD. I am not aware of any others.
Actually.
I won't even delve on the historical errors in the gospels nor reductio ad absurdums nor extra-biblical gospels, I will leave those arguments for other threads.