In the context of your question, "all this" means many things taken as a single whole. "All these" means many things as part of the whole; not everything.
For example I might be a boss pointing to a room full of boxes and say, "all of this needs to be moved into the next room." I can use the word 'this" because I mean everything. It is taken as a whole.
If I meant only some of the boxes, or if a mistake could be made if I'm not specific, I would point to specific boxes I wanted moved and say, "all of these need to be moved into the next room."
Another example I can think of is something I can picture from the movies: a rich guy looking out at all the land he owns with his son beside him. He waves his arm and says, "one day all this will belong to you." He means everything. The whole. Even if all his land is made up of individual items he would not say, "all these will be yours." It might be correct, but it also implies he is only talking about certain parts of the things in front of him. The son would ask, "which of these will be mine?"