I have no doubt that he would be hurt by not having won the Champions League, but if that really is a factor he would never have joined us in the first place when he was already at PSG.
I think Champions League contention was a significant factor, but not
the factor. If we compare what United would've offered relative to PSG: chance to play for a storied club, chance to win a league he's never won, chance to silence some doubts about his suitability to the physical game, chance to be reunited with Mourinho (who he has a good relationship with - and ironically, Internazionale won the CL immediately after Zlatan left - so there might be some lingering doubt in his mind in terms of
what could've been had I stuck with Mourinho), outside chance to win the CL in the next 2-3 years if things go well, a change of scenery (which could've done some good), etc. Plus, while Paris Saint-Germain are better than us in Europe and have a stronger team, over the last 3-ish years with Ibrahimović - they had stagnated at a QF type level - without
really threatening to go through the ceiling by beating one of the real big boys in the knockout rounds, so he might've felt that the cycle was ending and the time was up (a gamble from his end because he's now relying on José and United's finances to build a contender at a brisk pace).
Is there a credible CL challenger that would take him?
Maybe if he lowered his wage demands, told Raiola to STFU, and showed a level of workrate he didn't always in the past - a manager like Simeone at a club like Atlético would've taken a chance on him because #9 ahead of Griezmann is a fluid position for them. But he certainly wouldn't be a prominent player for some of the other teams because Benzema is a better fit with Ronaldo/Bale, Barcelona have Suárez (Zlatan wouldn't have gone back anyway), Bayern have Lewandowski, etc. Dunno about rejoining Juventus - doesn't seem to have the best relationship with them: