"Work" provides more than an economic trade. It's also fills many human needs: For many folks a stable routine and corresponding feeling of security, a set of constrained-yet-fulfilling social interactions, and a source of status and self-worth.
Virtual activities, whatever they are called and whether they pay or not, must fill many of the same needs.
For careers, first look to entertainment: Sports, politics, puzzles, drama, music, art, parties, tours, bake-offs, etc. Each needs real human minds to create, organize, advertise, manage events, and provide a myriad of support roles that cannot be done by NPCs. An NPC can serve virtual drinks at a party, but cannot plan which drinks match the party's theme.
In many industries, the real money --and many of the best jobs-- are in providing support and/or infrastructure. A music or sport star shines brightly for only few years, but their agents and bankers and lawyers make a happy living for decades while stars come and go. To be a star in the virtual world requires many of the same services. Advertising is a driver of today's real entertainment internet, and that industry's need for creativity and a wide variety of skills won't change.