it is confusing what he means in the introduction regarding piety. He starts and ends his book talking about piety, although "piety" is merely one of the traits listed in the ladder of ascent.
Here is an excerpt of a commentary from Rabbi Yechezkel Sarna on the Mesilas Yesharim which explains what it is doing in the introduction of the book (I am answering this based on my understanding of your comments):
Rabeinu called his book "the Path of the Just", not "the Path of the
Pious" despite that he opens his book with the "foundation of piety"
(in ch.1) and in the final section of the book says: "I did not finish
in this book all the principles of piety", and then goes on to
elaborate on "piety" until the end of the book. Likewise, in the
introduction, he speaks at length on piety, and so too in many
chapters of the book besides the chapters specifically designated to
the trait of piety.
It seems because piety is not the final end asked of man. It is merely
a level among the levels counted in the Beraitha of Rabbi Pinchas ben
Yair which Rabeinu in his book expounds one by one. The final end
asked of man is that he be "Yashar" Just/Upright as G-d made him. As
written "G-d has made man upright [but they have sought out many
schemes]..." (Eccl. 7:29).
Our sages asked (Avodah Zara 25a): "why is [Genesis] called the 'Book
of the Just'? Because it is the book of the Just, namely the holy
forefathers (Abraham, Isaac, Jacob) which are called Just." Hence,
this is what is asked of man. As written in Tana d'Bei Eliyahu, "a man
is obligated to say 'when will my deeds reach those of my forefathers,
Abraham, Isaac, and Jacob'". And that which Rabeinu speaks about the
trait of piety, opening and sealing his book with it, this is because
he wages a massive war campaign against the "Mitchasdim" (false pious)
in this book which are "most people" who consider many customs and many ways as
"piety" but they are only fallacious piety... and there is no trait of
all the traits counted by Rabbi Pinchas ben Yair of whom there is more
misunderstanding than piety. But what G-d asks of us is to that we be
Yesharim (upright/just). For this includes all the traits. For a
person who lacks even one of them is already not considered Yashar. Rather
he is among those who "sought out many schemes".
from Mesilas Yesharim with english commentaries here.
original hebrew of Rabbi Sarna is at www.hebrewbooks.org/41769 in the hakdama