Matthew 24:14 is not a commission to anybody in particular—not to the first-century apostles nor to anyone else. It is simply a statement of fact by Jesus Christ, prophesying that the gospel will be preached in all the world as a witness and then will the end come.
Matthew 24:14 and Matthew 28:19-20 are not synonymous. In the latter verses, though preaching as a witness is included within the scope of the commission, it actually places more emphasis upon the entire process of conversion, feeding, growing, and overcoming than merely witnessing, as in Matthew 24:14. The key word here is "process."
"Teach" is not wrong as long as we understand that it implies a process. All the teaching required to make a disciple cannot occur merely in making a witness. There are major differences between the two. At best, preaching the gospel to the world begins the process of teaching. Disciples are created through steady feeding, a believing response in those who hear combined with overcoming.
The second factor appears in verse 20: "Observe all things whatsoever I have commanded you." The key here is "all things." That cannot be done merely through a witness. As we are learning, observing all things is a lifelong project requiring the structure of a church. This is the reason why the church exists.
What is being emphasized in verses 19-20, though witnessing is included in it, is the feeding of the flock because it is the called, the elect—God's children—who are His greatest concern. These are the ones who are being prepared for the Kingdom of God. It takes a great deal of feeding and experiences with God for Christ to be formed in us.