There's a lot more to John the Baptist's quoting from Isaiah 40 about Christ's coming...
Luke 3:2-6
2 Annas and Caiaphas being the high priests, the word of God came unto John the son of Zacharias in the wilderness.
3 And he came into all the country about Jordan, preaching the baptism of repentance for the remission of sins;
4 As it is written in the book of the words of Esaias the prophet, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make His paths straight.
5 Every valley shall be filled, and every mountain and hill shall be brought low; and the crooked shall be made straight, and the rough ways shall be made smooth;
6 And all flesh shall see the salvation of God.
(KJV)
The part in underline John quoted from Isaiah 40. And it has a dual time application, once for Christ's first coming before the Jews would refuse Him, and then the rest of it in final at Christ's second coming to establish His literal Kingdom here on earth with a rod of iron, ruling over all nations. If Judah in Jerusalem had accepted Jesus of Nazareth as God The Saviour at His first coming, then the Kingdom would have come then.
So the following prophecy is still not yet complete...
Isa 40:1-11
1 Comfort ye, comfort ye My people, saith your God.
2 Speak ye comfortably to Jerusalem, and cry unto her, that her warfare is accomplished, that her iniquity is pardoned: for she hath received of the LORD's hand double for all her sins.
Is warfare already accomplished for Jerusalem today? No. There's still yet a great sin to come upon Jerusalem for the days just prior to Christ's second coming. It involves the harlot working of Rev.17. And then the battle of Armageddon is still yet. This should cause us to look further as to why John quoted this for Christ's first coming. The Way of The LORD through Christ is the part of this prophecy that was fulfilled at Christ's first coming. But, the rest of it is set for Christ's second coming which is still future.
3 The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God.
4 Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain:
This Isaiah 40 Scripture is applied to both of Christ's advents. But the detail given is especially about His second advent (coming) yet to occur.
Isa 11:16
16 And there shall be an highway for the remnant of His people, which shall be left, from Assyria; like as it was to Israel in the day that he came up out of the land of Egypt.
(KJV)
Isa 43:19
19 Behold, I will do a new thing; now it shall spring forth; shall ye not know it? I will even make a way in the wilderness, and rivers in the desert.
(KJV)
Isa 19:22-25
22 And the LORD shall smite Egypt: he shall smite and heal it: and they shall return even to the LORD, and He shall be intreated of them, and shall heal them.
23 In that day shall there be a highway out of Egypt to Assyria, and the Assyrian shall come into Egypt, and the Egyptian into Assyria, and the Egyptians shall serve with the Assyrians.
24 In that day shall Israel be the third with Egypt and with Assyria, even a blessing in the midst of the land:
25 Whom the LORD of hosts shall bless, saying, "Blessed be Egypt My people, and Assyria the work of My hands, and Israel Mine inheritance."
(KJV)
There's going to be a literal Highway in that time of His future Kingdom on earth spanning from Egypt, through the land of Israel, and up into Assyria. The deserts in those areas are literally going to blossum, most likely from God's River per Ezekiel 47 that will flow eastward out of His House in the future Millennium Jerusalem under Christ Jesus.
Isaiah 40:5 And the glory of the LORD shall be revealed, and all flesh shall see it together: for the mouth of the LORD hath spoken it.
There it is, a dual prophecy involving both of Christ's advents. That "glory" has a special meaning for what was manifested at Christ's first coming, and also what will manifest on eath at His second coming.
Isa.40:6 The voice said, Cry. And he said, What shall I cry? All flesh is grass, and all the goodliness thereof is as the flower of the field:
7 The grass withereth, the flower fadeth: because the spirit of the LORD bloweth upon it: surely the people is grass.
8 The grass withereth, the flower fadeth: but the word of our God shall stand for ever.
Our flesh bodies are like grass, and when God releases the four winds upon the earth at the "last trump" when His vengeance occurs at Christ's second coming, this will serve as the ultimate meaning of flesh as grass; yet The Word of God shall still stand, even forever.
Isa.40:9 O Zion, that bringest good tidings, get thee up into the high mountain; O Jerusalem, that bringest good tidings, lift up thy voice with strength; lift it up, be not afraid; say unto the cities of Judah, Behold your God!
Boy, now that really applied to Christ's first coming to Jerusalem. But it's also for His second coming also.
Isa.40:10 Behold, the Lord GOD will come with strong hand, and His arm shall rule for Him: behold, his reward is with him, and his work before him.
11 He shall feed his flock like a shepherd: he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young.
(KJV)
These two verses above are definitely for the time of Christ's future reign with a "strong hand". That gathering applied to the time of His first coming, to separate His called out ones, and it also applies to their future gathering by Christ at His second coming.