“ In the same way as we have obeyed Moses in all things, so will we obey you. Only Yahweh your God be with you, as He was with Moses.”
This was rather a rosy view of the relationship that they had had with Moses. They had not always been quite so responsive. But in general it was true. It would of course be much easier to maintain this unity and response when they were moving forwards towards a goal than when, having reached the goal, they had all settled down in different places. That was one reason why the gatherings at the central sanctuary would be so vital. It was to renew their central goal. But here the intention was good. They would obey him as they should have obeyed Moses.
But the proviso was that Joshua should prove himself YHWH's man, and that would be demonstrated by success, the final proof that YHWH was with him. It was not that they doubted that He would be. It was a statement of confidence. They were indicating that really they were committing themselves to YHWH, and to Joshua because he was YHWH's man. It is ever God Who must be central in our thoughts. Men are but His servants.