Greetings Laura, and blessings in Christ.
I'm not positive that being an eyewitness to His resurrection is an absolute requirement. And I think he asks a pertinent question in the following:
We know additional apostles besides these men exist because Christ, after His ascension, appointed “some as apostles, some as prophets, some as evangelists, and some as pastors and teachers . . . until we all attain to the unity of the faith and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ” (Ephesians 4:11-13). Are we there yet? Have we all attained to the unity of the faith? Or mature manhood? Or the fullness of Christ? Clearly, the apostolic ministry will continue until Christ returns!
I think the church, i.e. the true Spirit-filled church, is eventually going to arrive at this place one day. I think the end-times will bring severe trials, sufferings and persecutions, and these will help purify the church from the corruptions we see today, and cause her to led into ever-increasing intimacy with the Lord. Through this process, many false doctrines and interpretations that have divided the church and still keep her divided will be burned away, and through the returning ministry of the Holy Spirit Himself through Divine utterance and the prophetic gifts, the church will finally arrive at this place of possessing a true unity of the faith in Christ. As scriptures says, by the time He is returning, the church will have "made herself ready" (Revelation 19:7), and scripture implies elsewhere that this will be when she has become fully purified and "clothed in white." Paul said this was in fact his purpose (and by extension the purpose of the prophets, apostles, teachers, pastors, evangelists etc):
To the Corinthians:
2 I am jealous for you with a godly jealousy. I promised you to one husband, to Christ, so that I might present you as a pure virgin to him. 3 But I am afraid that just as Eve was deceived by the serpent’s cunning, your minds may somehow be led astray from your sincere and pure devotion to Christ. (2 Corinthians 11:2-3)
To the Ephesians:
21 Submit to one another out of reverence for Christ. 22 Wives, submit yourselves to your own husbands as you do to the Lord. 23 For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior. 24 Now as the church submits to Christ, so also wives should submit to their husbands in everything. 25 Husbands, love your wives, just as Christ loved the church and gave himself up for her 26 to make her holy, cleansing her by the washing with water through the word, 27 and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless. 28 In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself. 29 After all, no one ever hated their own body, but they feed and care for their body, just as Christ does the church— 30 for we are members of his body. 31 “For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.” 32 This is a profound mystery—but I am talking about Christ and the church.
Paul referred to this as a "profound mystery" because the analogy suggested there existed the potential to become completely purified in Him and "one flesh" with Him, if His Spirit so fully embodied us that we manifested His purity and power in every way. This what Paul was desiring in telling the Galatians he wanted to present them to Christ as a "pure virgin." Revelation 19:8 suggests the church will one day finally get there, for the "white garments" she will be clothed in will be the righteousness of the saints:
7 Let us be glad and rejoice and give Him glory, for the marriage of the Lamb has come, and His wife has made herself ready.” 8 And to her it was granted to be arrayed in fine linen, clean and bright, for the fine linen is the righteous acts of the saints. (Revelation 19:7-8 NKJV).