In 1 Samuel 13:13 it implies that if Saul had not acted impatiently but had waited for Samuel to arrive and perform the sacrifice “the Lord would have established your (Saul’s) kingdom over Israel forever”. That being the case, wouldn’t it also follow that the Messiah would have been a descendant of Saul (a Benjamite) which conflicts with previous prophecies that the Messiah would be from the tribe of Judah?
God knows the end from the beginning (Isaiah 46:10). None of His prophecies are contingent upon the actions of men because He already knows what our actions will be. Having told Jacob that the Messiah would come from the tribe of Judah (Genesis 49:10) and then having Samuel tell Saul he could have had the kingdom for all time (1 Sam. 13:13), only seems like a contradiction to us because we can’t see the future. Before God promised that the Messiah would come from Judah, He had foreseen that Saul would be a disobedient king and would forfeit the kingdom, and had already made provisions for the tribe of Judah to be the Messianic line.
We can see a similar situation in Genesis 15:12-16 where God promised the land of the Amorites to Abraham, but said he couldn’t have it for 400 years because “the sin of the Amorites has not yet reached its full measure.” Most scholars see this as an indication that God had given the Amorites 400 years to return to Him (all mankind originally believed in God). He knew they wouldn’t do so but had determined to give them the time anyway, after which He would use Israel as His instrument of judgment against them (Lev.18:24-28). Therefore He felt free to promise the land to Abraham 400 years in advance.