I've been thinking about what Jay and Kay's 2026 present would be like. Kay is so old at this point he can't possibly be an agent anymore, he's 86. And I don't think he would want to retire, he already tried to have a civilian life, he remembers that feeling of uncertainty and loneliness, he doesn't want to go back to that. So he decides he's going to just die on the job. He's old, he's slow, his reflexes aren't what they used to be, it's bound to happen and he's fine with that.
You know who isn't fine with that? Jay, the guy who spent the third movie trying to bring him back from the dead because he couldn't even comprehend a reality where Kay isn't alongside him (and to avoid an invasion, yes). So Jay starts looking out for Kay even more than ever before, even more than he looks out for himself. He isn't dumb, he knows his partner is old, he knows it's probably only a matter of time before something happens to Kay, but Jay is determined to keep him alive as long as he can, even if that kills him.
Kay notices. He obviously does, Jay is constantly two steps away from death because he keeps getting between Kay and danger. And then Jay actually almost dies. And it's different. It's not their usual "wow that was so close" situation, it's an "actually got a serious injury that will have repercussions on his health form now on" situation. And Kay realizes things need to change. Jay will always try to protect him, he won't let him die. So Kay takes him to that bench form the first movie (the "imagine what you'll know tomorrow" one) hands him his neutralizer and tells Jay he's retiring, because he can't keep being a men in black knowing it puts him in danger.
Jay protests. A neuralized Kay is better than a dead Kay, logically, but in the end it's all the same. He wants Kay by his side, he wants his partner. He doesn't want to have to look up at the sky, wondering if the only person out there who ever understood him is doing the same. He's about to start a fight when a thought comes his way. He's not exactly young anymore (57 or 61, depending on the continuity you're running with) and he doesn't want to spend one more day in mib trying to fill the Kay shaped hole that will inevitably be there.
It's simple really, if you think about it. They both retire. As far as I understand it, neuralization is a pretty loose concept. You can erase everything about a topic, but you can also erase only certain things. They erase everything about mib, except for one another. They could leave messages for themselves filling in some blanks as how you would do it if you neuralized some else. "You just saw x, it's actually y, you will do z."
They do it. They go back to civilian life. I have no idea what a normal life would look like for them, to be honest. Kevin seems pretty happy to work a normal job, provided he's surrounded by aliens and he gets to help people, even if it is with mundane things. The last job James ever had was filled with adrenaline and emotion, but that was way back then, when he was a young man looking for purpose and a place to belong. He knows where he belongs now: next to his partner, in their backyard after a long day, sitting side by side in two busted lawn chairs, holding hands and looking at the sky. Only they don't wonder if there's anyone out there that understands what it feels to be alone, because they're not.