The defining of Macbeth as a successful tragedy depends upon the definition of tragedy one uses.
First, according to one definition, a tragedy is
a dramatic composition, often in verse, dealing with a serious or somber theme, typically that of a great person destined through a flaw of character or conflict with some overpowering force, as fate or society, to downfall or destruction.
Under this definition, one could consider Macbeth a successful tragedy based upon the fact that the play is a dramatic composition, completed using verse, which deals with a serious theme (murder, tyranny, and the errs of man/woman). Not only that, Macbeth's tragic flaw (hamartia- Macbeth's being his ambition) leads to his downfall and destruction.
On the other hand, tragedy
can be defined as
a form of drama based on human suffering that invokes in its audience an accompanying catharsis or pleasure in the viewing.
Based upon this definition, one could argue if Macbeth is a successful tragedy. If the play fails to evoke the readers emotionally, then one (that individual reader) would not consider the play a successful tragedy. Apart from that, if a reader becomes emotional, then they would consider the play a successful tragedy.
Essentially, the success of the play, regarding its denotation as a successful tragedy, is based upon both the definition applied and the reader's response.