jQuery is like your cool older first boyfriend who you pretend is a good decision even though he doesn't understand your taste in movies and is jealous of your guy friends. Cut to six months later and you're still together, because honestly what's the alternative, pure JS and CSS? Nah, fuck that. You want fancy and convenient.
The problem is you're a grown up now. You write grown up code with grown up frontend frameworks, and jQuery boyfriend has become this glaringly incongruous thing that you keep around because you met his parents and don't want to make him cry.
Here are a few reasons to move on to greener pastures:
1. One less thing to load and possibly slow down UI performance
2. The DOM API is just fine for little things
Many of us, myself included, never bothered properly learning the DOM API. Can you blame us? jQuery is often cleaner, easier, more robust and takes care of cross browser issues. However, unless you require IE7 support, the selectors available to you in the Web API are generally adequate.
3. The JS spec keeps evolving
4. Manipulating the DOM can be an antipattern
With front end libraries like React, Angular and Vue, direct DOM manipulation is unnecessary and kind of gross. It really comes down to separation of concerns and maintainability. If you're going for modularity and scalability, jQuery ends up just being one more thing to worry about and introduce bugs when you make changes to your code.
5. You don't need $.ajax()
When to drunk text
At this point jQuery is probably happily married to someone who drinks white wine and thinks astrology is real. However, if you're working on a small project or plugin, and performance doesn't really matter, there's still a case for giving him a ring.
P.S. John Resig is a fucking god, and jQuery is one of the most mindblowingly impressive and influential contributions to web.