Jesus confirmed the scribes" teaching about Elijah coming, but He said another factor needed consideration. John the Baptist"s ministry had been a success as far as it had gone (cf. Matthew 3:5-6; Matthew 14:5), but he had "restored all things" to only a limited degree. The scribes perceived the ministry of Messiah"s forerunner correctly, but they did not realize that John the Baptist had been that forerunner ( Matthew 11:10). Elijah had already come in John the Baptist. However, Israel"s leaders had rejected him, and he had died without accomplishing the complete restoration of Israel. John had not fulfilled his mission but died doing so. Likewise Jesus would die at His enemies" hands without fulfilling His mission of establishing the kingdom. John had restored all things as much as he could and yet died. Jesus, too, would fulfill His mission as much as He could and yet die. This was the answer to the disciples" question. [source][source][source]
"A suffering Forerunner is to be followed by a suffering Messiah." [1][source]
"In other words, just as the messianic forerunner"s coming had two phases: John the Baptizer (one to suffer and die), and Elijah the Prophet (one of restoration and glory), so also would the Messiah"s coming. The response to the forerunner foreshadowed the response to the Messiah and necessitated the postponement of the fulfillment specifically promised to national Israel." [2][source]
God predicted through Malachi that a Jewish revival would precede Messiah"s kingdom ( Malachi 4:5-6), and the revival did not come. Consequently that revival and the kingdom must still be future. [source][source][source]