Why was the kingdom offer to Israel only a valid offer and not something God expected to happen?
The kingdom offer was valid because it was genuinely conditioned on every Jew repenting and being baptized, after which Christ would lift the curse and bring in the kingdom. Yet God foreknew Israel would not fulfill that prerequisite, so He knew He would turn to the Gentiles without Israel.
Les teaches that the offer required every Jew to repent and follow with water baptism, as Acts 2:38 was still appropriate. If Israel had done so, Christ would have ushered in the tribulation, lifted the curse, and brought in the kingdom, and Israel could have evangelized the nations. But it was only a valid offer, because God knew Israel wouldn't do it. God therefore knew He would go to the Gentiles without Israel. From the crucifixion to the temple's destruction by Rome was 40 years, and Acts covers that period. At the end, Israel remained adamant in unbelief, so God permitted Rome to destroy the city and temple and scatter Israel among the nations.