This is a great question, and cuts right to the beginning of the purpose of creation.
Firstly, the Ramchal himself explains this by stating in Derech Hashem (1:2:1-2) that Hashem wants to give people the best good, which is Himself, and therefore the only way to achieve that is for us to be like Him in a certain respect: i.e. we are to be the masters of our own good. This means that we must have free will and the environment to exercise that free will that gives us those opportunities to fairly achieve that goodness through our own choice, toil and effort.
This free will must be truly free, which means there should be a fair chance that someone can make the wrong choice. Ramchal elaborates later on that after eating the fruit, the balance became a lot worse, and now the majority of our inclination will be towards selfishness and evil.
Tanya explains giving Himself means Hashem desires a "dwelling among the lowest beings". We are serving Him in fulfilling this desire by partnering with Him to build a true dwelling with us. A true dwelling is unity between us and Him, and each other, forever, as Ramchal states:
שתכלית כל הענין הוא לקבץ קיבוץ שלמים שיהיו ראוים ליקבע לנצח בהתדבקותו
The purpose of all this is to gather a perfected group fit to be together
with Him Yitbarach, in dvekut forever
This intimate oneness is so good that it's purely His infinite kindness and generosity that He gives us an opportunity to have it with Him; purely our priviledge. It's pure kindness that it is as easy as it is, even if it's very hard (or is it? See next paragraph). The "punishment" is the extreme regret one has on the squandered opportunities one lost in achieving that Oneness for himself, at the end of days when it's too late to try again.
That He desires this is our great benefit, but the point of the Tanya is that it is His desire; His desire to dwell with us. This answers your question because, if it's for Him, He will get what He wants. So we see many themes in many Torah sources that demonstrate how Hashem navigates our lives and history with mercy, so that far more of us are saved than meets the eye. As it says (Shmuel 2:14:14):
ולא־ישא אלהים נפש וחשב מחשבות לבלתי ידח ממנו נדח
...neither does God take [away] life, but devises means, that
none of us be banished. [Samuel II 14:14]