What do you think the speaker means when he says that he "could wish" his days to be bound to "natural piety" ?why he doesn't he say that he is sure that his days will be bound to it ?
Wordsworth is referring to the past pattern of his life here. Born in an area of outstanding natural beauty in the lake district of northern england, he could not help but be moved by the stunning beauty of natural wonders such as rainbows. So he spells out for us the following stages of man's life and how this respect for creation has remained a constant with him throughout. He sees no reason why this 'natural piety' should change in the future, and is hopeful and optimistic about the part of his life that is still coming. However, none of us can see into the future and so there is always a smidgeon of doubt. He shared this appreciation of Nature/God with his sister and prayed for a similar outcome for her in his Tintern Abbey poem.
I think that he does not say that he is sure his dies will be bound to it because he is not sure they will be. He hopes that they will, but he is not sure.
You can see that in lines 5 and 6. He is saying there that he wants to be the same when he is old as he was when he was young -- if not, he would rather die.
But he is not absolutely sure that things will work out this way. Wordsworth did not write this poem as an old man, so he could not be certain that he would (as he hopes) be able to maintain this connection with his youth as he aged.