The passage says that Jewish practices will change so much that proselytes from gentiles will come.
It does not just say that the proselytizes will come.
Dropping that first part out about mass change within judaism as a pre-requisite makes it seem that we are talking about jewish teachings as they exist today.
So what are the prophesied changes:
Zechariah 8:19 (KJV 1900)
19 Thus saith the LORD of hosts; The fast of the fourth month, and the
fast of the fifth, and the fast of the seventh, and the fast of the
tenth, shall be to the house of Judah joy and gladness, and cheerful
feasts; therefore love the truth and peace
- The fasts of Tammuz and Ab (in the fourth and fifth month) will become celebrations and days of joy.
These fasts mourn the destruction of the temple, the death of David's child, Moses breaking of the tablets of the law, and the loss of the land. So celebrating these events would require that the new judaism is no longer attached to the land, or the service of the physical temple, or the giving of the law, or to bloodlines, but views these historical catastrophes as blessings and occasions to celebrate because it freed them to worship God only in a different way. In other words, it points towards the spiritualizing of jewish practice.
E.g. Paul interpreted this to mean that jews would recognize Jesus as messiah, abandon the desire for temple worship and observance of the law, and then join with the gentile believers in obtaining many converts from all over the world.
As it is very hard for a religion that views itself as tied to things like bloodlines and physical geographies to win many international converts, and judaism isn't going to be able to drop those attachments until it stops reading things literally. But it is those physical attachments that explain rejection of Jesus as messiah, as they were looking for an earthy king and an earthly reign that can be seen with the eyes. Dropping those attachments to things of the flesh and the tablets of the law is what celebrating Tammuz and Ab suggests.
Now you don't need to believe Paul's interpretation as you can have your own, but you do need to understand why the fasts mourning these events would become celebrations of joy. This requires the reader to produce some theory as to what the changes in judaism would be that would lead them to win converts from all over the world similar to how other more international religions do now.
But there are more promised changes, namely God himself returning to Jerusalem
Thus saith the LORD; I am returned unto Zion,
And will dwell in the midst of Jerusalem:
And Jerusalem shall be called a city of truth;
And the mountain of the LORD of hosts the holy mountain.
Now ostensibly there was a fleshly fulfillment of this prophecy, because the temple was rebuilt and the pharisees had converts in the Roman Empire. So in some sense, we can say that this was partly fulfilled in the flesh during the Second Temple era.
But of course it was not fully fulfilled because the fasts were never celebrated as moments of joy and the temple was destroyed. So what does God returning to Zion mean? Again, John saw the New Jerusalem descending as a bride adorned for her husband, and associated the new Jerusalem with the christian church, and thus a joining of judaism to the church. You can have a different interpretation if you want, but some interpretation is needed for what these pre-requisite changes in judaism would be.