I mean, let me count the ways:
- Eren killed 80% of humanity with the rumbling...so no one really stopped him and saved the world. Armin et al's "we saved the world by stopping the Attack Titan" business is at least 80% bullshit.
- Eldia's still in hyper aggressive "kill them before they kill us" mode as a country - so the chain of hatred, fear, revenge etc was absolutely not broken.
- How did Ymir meeting Mikasa in the Paths alter Ymir's love for old King Fritz and her desire to continue Fritz's will (to continue giving birth to an endless Titan race/empire)? What about meeting Mikasa resulted in this change? The only character development we've seen from Mikasa is 1 - denial that her superhuman strength is genetically programmed and 2 - being really sad that she has to kill Eren. No significant changes in her other than "Whelp, my crush has become the antichrist, guess I gotta kill him".
- Eren transcends time, "Butterfly Effect" style, and can control Titans through the coordinate THROUGHOUT HISTORY, as revealed in his "I had to make Dina Fritz's Titan eat my mom instead of Bertholdt" confession. With that godlike power...he did nothing to change this weaksauce fate.
Literally the only thing Eren accomplished, at all, was putting an end to Titans and the world's fear of Eldians as man-eating monsters. Now the world fears Eldians as genocidal assholes bent on conquest via conventional means. Oh, and he saved his buddies, a worthless brat, and the other titan shifters, by letting them (letting them? really? seems he went all-out to kill them personally) defeat him and become the heroes that stopped the apocalypse...when it was 80% complete anyway.
THAT is why this was a weak end.
1. Well yes, but actually no.
If it weren't for the "shifters", genocide would have went from the 80% to the 110%.
They did what they could and managed to stop it, it is a win, regardless of the numbers slain.
Regardless of what was said by Eren in the Paths to them, that does not influence what the world will think of them.
They saved the world from the absolute, certain genocide, it is a win for the survivors regardless of the gargantuan piles of guts smashed to the soil.
2. Eren, above all else, values freedom. True, absolute freedom.
Also, he couldn't have erased or altered the memories of the Paradis residents, as the same scenario (world hating Paradis) would have continued.
Also, Paradis is building their army in order to defend, not to conquer.
Eren leveled the playing grounds, if one can say so.
"Chain of hatred" shall exist as long as humans are alive.
Don't forget, AoT is built upon complexities of human emotion, not idealism, this is not a fairy tale.
3. Ymir didn't meet Mikasa in the Paths, she saw the answer to her pain and conondrum through her actions.
Her love and devotion was IRRATIONAL, but having gained the godhood as a child, whan can you expect from someone like that..?
Also, time does not exist in the Paths, and most likely, you need to have both the Founder and Attack titan powers to "influence the past" and "run simulations", which is why she didn't get her "answer" up until Eren.
As for the "answer", one can only speculate.
Doing what must be done regardless of your own selfish (which love, in some way, is) desire, moving on or something else.
From what I understand, Mikasa gave the so much needed rest for Eren, because he was dead inside from the moment he kissed Historia's hand.
4. As mentioned above, Eren doesn't want to mendle with free will, or so it seems and would fit his character.
Also, he can only influence Eldians, and even then it is a question to how much he can do so. I doubt he would mendle with anything past the Owl (would risk of changing too much, maybe even his friends and etc. not being born). That is important because the hate towards Paradis already exists, so he can't do much either way, the only thing he can do is "ending it one way shape or form", to which even he doesn't know the answer.
Eren is a human, shackled by his omnipresent God-like powers. He is flawed, just like all humans are. It's too much for one to bear...
I mean there are people who can't wait to die as they lived for too long, and having the space where all IS, where you have literally, from your perspective, eternity...?
Who knows how much he spent there.
Years? Decades? Eons..?
After all, he is a HUMAN. Not a God.