but I think this is weakness , what do you think ?

This change was made for these reasons: (a) to enable to chart to be displayed in a dynamic report instead of the tree view, allowing smart filters and comparisons; (b) to simplify the concepts needed when setting up accounts. Odoo has seen many examples of databases setup incorrectly - without parents. Users also didn't understand Root View. Additionally, other Accounting systems don't use this same concept; (c) Because filtering for a class of accounts, or a search, is quicker than expanding and navigating the tree. Optional TAGS have taken the place of Parents, and the way to browse them is now via the interactive reports.