I practiced the Goenka’s technique a number of years, and found it to be quite an oversimplification (and at times misrepresentation) of Buddha Dhamma. The benefits, as you pointed out, are the temporary relief one gets after attending a retreat.
First, the verse you quoted:
All saṅkhāras are impermanent.
When you perceive this with true insight
then you become detached from suffering;
this is the path of purification.
… is the translation of the Pali verse:
“Sabbe saṅkhārā aniccā” ti,
yadā paññāya passati;
Atha nibbindati dukkhe,
esa maggo visuddhiyā.”
…which is Dhammapada 277 and is explained in:
This post is about Anuloma Khanti and Sammattaniyāma.
At the start of the thread, you wrote:
“…Another thought strongly came to mind, “Nibbana is the only way out.””
It sounds like you may have realized Sammattaniyāma, and if that is the case then it is great!
The oversimplification is that the method focuses on just bodily sensations: dukha vedana, sukha vedana, adukhamasukha vedana. It is explained that these sensations are manifestations of all mental phenomena, especially sankharā.
Very briefly, the “vipassana” part of this technique involves moving attention from head to toes, noticing the sensation in each part of the body, and observing that sensation with a mindset of equanimity. (The “anapana” part of the technique is taken to be breath meditation).
For those who are following this method, and this is all you do, then do not be surprised that after years and years of practice your gati seems to stay the same.