Moses had worked very hard. Israel was going in to the land but they wouldn't be having a rest right away. They had to conquer and then build and plant and settle and put everything in order. Moses was an old man and full of days. Building and planting are more work. He was ready for a true eternal inheritance. Still, if it had been me up there surveying the landscape, I would have been muttering.
It makes more sense, or comes full circle....what is the word I'm grasping for....the sense of disappointment dissipates when you come to the Ascension. Jesus does a full work of atonement and redemption and oblation and satisfaction. He brings his sheep safely through. He conquers evil. He does what he came to do and then, as he rises up on the cloud, surveying the landscape, it's not a picture of muttering but of relief and joy to be going to sit at the right hand of the Father. It's the disciples who stand around covered in disappointment. What do you mean we have to go on without you?
But Jesus going away was the greatest mercy. Sitting at the right hand of the Father, interceding for us, living in us through the Holy Spirit, conquering and building and settling in and putting everything in order--he doesn't rest, though his glory must be a rest for us.
Either way it's mercy, and with it a measure of grace.