Is there a reason why bulk ferment is not completed in the banneton?
We know that the proofier the dough, the more likely we will degas it during shaping. Why not just shape it shortly after the last stretch and fold and the let it finish rising in the banneton? Is there a flaw to this idea that I'm not seeing?
I'm not sure I understand how proofing in the banneton can overproof easier than bulk fermenting in a different container, if that is what you are saying.
Let's say it's an ordinary plain white loaf. One can allow the dough to rise, knock back, shape and rise again. Not going into too much detail and to make it easier to understand let's say the bulk ferment can be left to double, then it's shaped and allowed to rise a second time.
If it's shaped straight away or early then it will have to be baked once it's risen to a certain height which is about just under doubled. So a bulk ferment, knock back, shape and proof again allows it to ferment for longer. Shape too soon or straight away the ferment will be shorter.
I have seen plenty of yeasted dough recipes calling for letting the dough double in size, knock down, preshape and shape, and then rise again. I don't think I have ever seen a sourdough recipe calling for a dough to be knocked down. Is this something I should be doing? If I let my dough rise back up to the original volume, how will the crumb structure look compared to only letting it rise once (to that same volume).
Will be a "knocking back". Sourdough tends to focus a lot in the crumb as well so we don't always hear of knocking back like we do when it comes to yeasted sandwich loaves. However there's still some degassing with the folding and shaping strengthening the dough further for a better rise. Allowing it to bulk ferment and then a second ferment will improve the flavour since it's had more time and a better structure due to the shaping after the bulk ferment with a more even crumb. But bear in mind you don't have to stick to this. By all means knock back if you wish. And some recipes call for one ferment done very well with a very gentle shaping and then baking. When it comes to bread there are many methods but that is the reasoning for two ferments. In a nutshell it can be done in a loaf pan but that isn't the bulk ferment.
In your example as soon as you shape and put it in the banneton it's now gone into final proofing rather than finishing off the bulk ferment. Tied into what banana said the total ferment time will now be reduced affecting the final loaf.
Dough can overproof and you will end up with a dense collapsed loaf. By doing the bulk ferment you can develop flavor without overproofing.
I'm not sure I understand how proofing in the banneton can overproof easier than bulk fermenting in a different container, if that is what you are saying.
Let's say it's an ordinary plain white loaf. One can allow the dough to rise, knock back, shape and rise again. Not going into too much detail and to make it easier to understand let's say the bulk ferment can be left to double, then it's shaped and allowed to rise a second time.
If it's shaped straight away or early then it will have to be baked once it's risen to a certain height which is about just under doubled. So a bulk ferment, knock back, shape and proof again allows it to ferment for longer. Shape too soon or straight away the ferment will be shorter.
I have seen plenty of yeasted dough recipes calling for letting the dough double in size, knock down, preshape and shape, and then rise again. I don't think I have ever seen a sourdough recipe calling for a dough to be knocked down. Is this something I should be doing? If I let my dough rise back up to the original volume, how will the crumb structure look compared to only letting it rise once (to that same volume).
Will be a "knocking back". Sourdough tends to focus a lot in the crumb as well so we don't always hear of knocking back like we do when it comes to yeasted sandwich loaves. However there's still some degassing with the folding and shaping strengthening the dough further for a better rise. Allowing it to bulk ferment and then a second ferment will improve the flavour since it's had more time and a better structure due to the shaping after the bulk ferment with a more even crumb. But bear in mind you don't have to stick to this. By all means knock back if you wish. And some recipes call for one ferment done very well with a very gentle shaping and then baking. When it comes to bread there are many methods but that is the reasoning for two ferments. In a nutshell it can be done in a loaf pan but that isn't the bulk ferment.
In your example as soon as you shape and put it in the banneton it's now gone into final proofing rather than finishing off the bulk ferment. Tied into what banana said the total ferment time will now be reduced affecting the final loaf.
Perhaps the discussion in this other thread helps?
https://www.thefreshloaf.com/node/62912/back-basics-why-rise-twice
Take a look at this link.
https://www.thefreshloaf.com/node/62981/pan-breads-simplified
After the OP, scroll through the entire post.
Please delete
Knead by hand to degas, shape, let it double, bake. Enjoy!