Also, don't infantilize Eui Joon. He was not a high schooler nor just starting university... He was finishing university, already working at the same time than studying, already having to be responsible for a sick relative, already living by himself, already knew his sexuality and comforatble with it and out of the closet to his friends, already on his middle twenties. He had little experience with dating, but it wasn't a predatory situation nor a situation in which ahjussi had to wait because the other guy was too young... Those are all excuses. It's actually ahjussi the one who was confused with his sexuality and hesitant at first.
The 3 years thing is obviously something the author prepared to make the story have forshadowing and eventually make the plot go full circle... But, from the characters perspective, that's not a real agreement nor a promise nor anything... It was just ahjussi impulsively showing that he had the determination to "wait for Joon"... Since Joon had been recently heartbroken by a jerk... But ultimately, ahjussi didn't kept that pseudo-promise, it was all talk. Ahjussi never waited, he just left.
*"Waiting for someone" in a romantic sense implies "going slow", starting as friends or in a more casual relationship before making things official... It implies getting to know each other better slowly, deepening your relationship, before comitement... It's not about being completely separated without any communication for a while, that's THE CONTRARY of "waiting for you".
The point: ahjussi had his reasons and I have compassion for him... I don't blame him for everything. However, he didn't left because his a good guy nor did leaving made him a good guy. Let's not lie. And ahjussi isn't narcissictic or abusive with his partners, but he's NOT "a walking green flag" or a saint neither. He has defects and makes mistakes and does have his own few toxic traits. He's human. But not a role model or anything.