Why was Joshua so pessimistic before he died?

The Israelites were optimistic about worshiping the Lord in Joshua 24:

16 So the people answered and said: “Far be it from us that we should forsake the Lord to serve other gods; 17for the Lord our God is He who brought us and our fathers up out of the land of Egypt, from the house of bondage, who did those great signs in our sight, and preserved us in all the way that we went and among all the people through whom we passed. 18 And the Lord drove out from before us all the people, including the Amorites who dwelt in the land. We also will serve the Lord, for He is our God.”

Click to expand...

But Joshua was pessimistic:

19 But Joshua said to the people, “You cannot serve the Lord, for He is a holy God. He is a jealous God; He will not forgive your transgressions nor your sins. 20 If you forsake the Lord and serve foreign gods, then He will turn and do you harm and consume you, after He has done you good.”

Click to expand...

Why did Joshua put down the enthusiasm of the people?

21 And the people said to Joshua, “No, but we will serve the Lord!”

Click to expand...

Why is Joshua so pessimistic before he dies?
Moses' feeling was similar before he died, Deuteronomy 31:

14 The Lord said to Moses, “Now the day of your death is near. Call Joshua and present yourselves at the tent of meeting, where I will commission him.”

Click to expand...

Joshua was personally with Moses before he died. Joshua felt what Moses felt.
The Lord commanded Moses:

19 “Now write down this song and teach it to the Israelites and have them sing it, so that it may be a witness for me against them.

Click to expand...

It was a song against the Israelites. That's among Moses' last words to the Israelites. Moses knew the bad news.

29 For I know that after my death you are sure to become utterly corrupt and to turn from the way I have commanded you. In days to come, disaster will fall on you because you will do evil in the sight of the Lord and arouse his anger by what your hands have made.”
30 And Moses recited the words of this song from beginning to end in the hearing of the whole assembly of Israel

Click to expand...

Deuteronomy 32:

16They made him jealous with their foreign gods
and angered him with their detestable idols

Click to expand...

They would commit apostasy.

23 “I will heap calamities on them
and spend my arrows against them.
24I will send wasting famine against them,
consuming pestilence and deadly plague;
I will send against them the fangs of wild beasts,
the venom of vipers that glide in the dust.
25In the street the sword will make them childless;
in their homes terror will reign.
The young men and young women will perish,
the infants and those with gray hair.

Click to expand...

Why does Joshua put down the enthusiasm of the people?
The people were focusing on the present good time. Joshua saw their evil future.

Jo555 said:

Curious as to why these minor details are important to you?

Click to expand...

Good question. That is how my brain works. I prefer to think in specific precision.