It is easy to imagine that the rich man of the story probably would not have invited the poor, or people from the highways, to his banquet had his original guests shown up. However, that is a hypothetical and uninformative as to God's intentions for Israel and the Gentiles.
All metaphors have limitations and some metaphors are meant to extend more deeply than others. The parable of the prodigal son has turned out to be quite deep, and many sermons have been preached over the years on all aspects of it.
This parable is a little different. Here, Jesus is not only acting as a teacher but also as a prophet. He exhorts the Pharisees to repentance, by likening them to the invited guests, but he also foretells what will happen starting in only a few years' time. The Pharisee establishment (the guests) will reject the Messiah of the Jewish people. Many of the lowly within Judaism, foreigners who live in Judea, and some Samaritans (the poor, blind, lame) will accept Him. And God will pour out His Spirit on them and on the faithful Gentiles (those on the highways).
Because this parable is a combination of moral teaching and prophecy, to ask what would have happened if the guests had come carries implications well beyond the parable itself. It is the same as to ask, if the Pharisee establishment had accepted Jesus as Messiah, would the Holy Spirit have been poured out on the Gentiles? I am not sure whether that question is even fully answerable. Many Christian teachers are of the opinion that the prophet Joel foretold this:
It will come about after this that I will pour out My Spirit on all
mankind; And your sons and daughters will prophesy, Your old men will
dream dreams, Your young men will see visions. Even on the male and
female servants I will pour out My Spirit in those days.
Accepting this meaning of Joel 2:28-29 for the moment, that would imply that by the time Jesus gave this parable, the written Word of God already contained foreknowledge of what would take place. Therefore, any other outcome was impossible. Jesus would be rejected in Jerusalem, and the Holy Spirit would be poured out on the faithful Gentiles.
But what if the events of Acts 10 and beyond are not what the prophet Joel was referring to? Even then a case can be made; the prophets Isaiah and Zechariah certainly foretold a rejected Messiah, and even the Abrahamic Covenant promises that the nation of Israel would serve as a means for God's blessings to reach the Gentiles (Genesis 12:3) - the Jewish people, in essence, will have a priestly role towards the world at large.
So in my opinion, even if it were possible that the guests (the Pharisees) appear at the banquet (relationship with God through Jesus the Messiah), the rich man (God the Father) might not have sent the servant (Jesus, the Apostles, and the Church) into the highways (the Gentile nations). But those people still would have somehow shared in the bounty a different way. But exactly how that might have happened is exceedingly hypothetical, and obviated by prophetic foresight in the written Word, and well beyond the scope of the parable.