The problem is that the Bible teaches its Laws are eternal. They do not change with time. Therefore, the miracles promised in John 14:12 are to be done eternally.
Actually, there are no miracles promised in John 14:12. You made that up. Here are four translations of the passage:
Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father.
Very truly I tell you, whoever believes in me will do the works I have been doing, and they will do even greater things than these, because I am going to the Father
“I tell you the truth, anyone who believes in me will do the same works I have done, and even greater works, because I am going to be with the Father.
“Truly, truly, I say to you, he who believes in me will also do the works that I do; and greater works than these will he do, because I go to the Father.
I may be wrong, but Jesus certainly could have substituted the word miracles for works in this passage, but he did not.
The question then becomes what did Jesus mean by works? In Matthew 25, Jesus talks about separating the sheep from the goats. He gives specific works that will figure into that separation. It can be argued that greater works have indeed been done (quantitatively) since Jesus, through the spread and workings of the church. Jesus really worked in a relatively small area during his earthly life.
Which brings me to another point. It's often argued that people who believe the Bible will pick and choose what is to be taken literally and what is to be taken as metaphor. This argument is used to try and undermine belief by saying you either take everything literally or you take nothing literally. That's an argument that's too clever by half because it assumes that one has to be mindless in order to be a believer in scripture.
When Jesus told the disciples "I will make you fishers of men" are we to think that he meant that we are to go out in boats and throw nets over swimmers?
In Matthew 5, When he said "you are the salt of the earth" are we to think that he meant we are literally Sodium Chloride (NaCl)?
When he said, also in Matthew 5, "You are the light of the world" and "let your light shine before men.." are we to think that he meant that we are to go out shining flashlights in front of people?