If the Scripture in which the eternal, unbreakable promises that God made to Israel can be denied, then the entire Bible has to be denied, including the sections of the New Testament that Christians believe detail the plan of salvation God provided to the world.
We don't want to be fraudulent, dishonest hypocrites, right? If God breaks His promises, and has broken His promises to His covenant people, the Jews, then He has broken His promises to Gentiles who mistakenly believe they are born again based on the information God has given in His Scriptures.
Where dispensationalists diminish their credibility is by stating that Israel does not have to come to faith in Jesus Christ to be saved.
No dipensationalist I've ever heard or read, with the possible exception of John Hagee and what he actually said has been debated, has ever claimed that there is different path to salvation for Israel apart from Jesus Christ.
And one of the many, many ways that those who cling to the demonic doctrine of replacement theology, in direct and open rebellion and contradiction to the Word of God, lose whatever credibility they may have left after defending replacement theology against all Scriptural proofs against it, is by claiming that God made absolutely no provision for the salvation of Old Testament Jews and Gentiles simply because He chose not to send Jesus Christ at that particular time.
No dipensationalist I've ever heard or read, with the possible exception of John Hagee and what he actually said has been debated, has ever claimed that there is different path to salvation for Israel apart from Jesus Christ.
Really?
During the Tribulation, those who come to Christ will not have eternal security as the Church does now. They will have to continually "earn" their salvation, so to speak.