Users are receiving threshold errors even we have indexed the columns

1. SharePoint farm 2013 enterprise.

2. Inside our team site we have a document library which had around 4,800 documents 2 month ago.

3. The DC at that time did not have any indexes define.

4. Since inside one of our list views we are showing all documents without folders as follow:-

5. So i were afraid that end-users will receive threshold error when the DC reaches the threshold limit (5,000), if they do a column-based filtering or sorting.

6. so i added all the available columns to be indexed and i run a full crawl:-

but today the document library reaches 5020 documents, and when end-users access the list view, they will be able to view all the documents without folders (since the list view is defined to sort the documents based on the ID), but if they try to do a column-based filtering/sorting they will get this error:-

so why the indexes did not work for me? as end-user are doing a column-based filtering based on an indexed column (for  example in the above picture the end-user try to do a filter based on the "ch" column which is a drop-down column which contain 2 options), also this will happen if they try to do a column-based filtering/sorting for the modified-by,created-by,created or modified fields...