Incorrect.
There is no such thing as level of effectiveness in D&D. You show me text from any, and I mean "any" book that says you must create an effective character. Now the DM can fiat his home brew however he see's fit, just like he can rule that only a fighter can choose greatsword.
"Characters are assumed to be between 3 and 5th level with a minimum of 3 magic weapons between them". "All published adventures will be use the Wealth by Level guidelines."
There are definitely levels of effectiveness in D&D. I think what you meant to say is there are no defined standard for minimal effectiveness within the rule set. So? This ends up being part of the social contract at the table.
If any character cannot meaningfully contribute then there should be in-game consequence.