James White often says "We stand on the shoulders of giants". And he's right. So we have a treasure of 2000 years of theologians, apologists, and commentators at our disposal. That is not to say we should blindly accept what they say, but we should consider their opinions, and make sure what they say is Biblical.
Now, since Calvinists don't usually use John 15 to "prove" eternal security, we have no obligation to prove our view from this passage. (Not all doctrines are taught in all verses.). But since no positive argument has been presented to show that this passage refutes eternal security, the bar isn't set very high.
I have a number of electronic commentaries in my collection, and the first one I turned to was the "People's NT Commentary", although it is not my favourite. But I did like what it said:
"2. Every branch in me that beareth not fruit he taketh away. As the husbandman cuts off the unfruitful branches of the vine, so the Father severs the unfruitful branches from his Son. Judas, an unfruitful branch which did not have the life of the Vine, had just been severed and had gone forth. Every branch that beareth fruit, he purgeth it. The husbandman prunes and dresses the branches in order that they may be more healthy and fruitful. The Father cleanses, purifies, frees from sin, all who become branches of the True Vine."
-- People's NT Commentary, John 15:2
It is cut off because it is not connected to the root, that's why it's cut off, as it doesn't have life. It's very similar to the parable of the soils, were some of the seeds appeared to have life (since they sprouted), but didn't really (since they didn't bear fruit. I've said it many times before... Having faith is not the "requirement" for salvation, it is the EVIDENCE of salvation. It is the "litmus test" of whether or not someone has eternal life.
It's all about appearance. Something or someone can "appear" to be saved, but not really be saved (1 John 2:19). It reminds me of the wheat and the tares. I'm told that tares look similar to wheat, they have the "appearance" of wheat, but they aren't.
My go-to is Dr. A.T. Robertson, a Greek scholar:
"2. Branch (klēma). Old word from klaō, to break, common in LXX for offshoots of the vine, in N.T. only here (verses 2-6), elsewhere in N.T. klados (Mark 4:32, etc.), also from klaō, both words meaning tender and easily broken parts. In me (en emoi). Two kinds of connexion with Christ as the vine (the merely cosmic which bears no fruit, the spiritual and vital which bears fruit). The fruitless (not bearing fruit, mē pheron karpon) the vine-dresser “takes away” (airei) or prunes away. Probably (Bernard) Jesus here refers to Judas."
-- A.T. Robertson, "Word Pictures of the New Testament", John 15:2