The D90 has relatively primitive around 20 year old AF technology.
The D90 was among the best enthusiast DSLR's available in its day. People made great photos - award-winning photos with that camera.
Let's not rewrite history to suit a contemporary bias. The impression given by the OP that DSLRs from the 2000s couldn't reliably autofocus is simply false. Autofocus technology wasn't as capable then as it is today. But that's a far cry from claiming that autofocus in the 2000s was incapable, that it couldn't be used or relied upon to make quality photos. That narrative is a myth.
Modern AF in all brand ML bodies is dramatically improved.
It's dramatically improved for certain types of photography. If you are a landscape enthusiast, there's zero reason a 2000s era DSLR like the D90 couldn't be used to achieve good focus on a scene. Macro photography, cityscapes, portraiture, night sky...DSLRs could be used and were used to make great photos. Their focus systems were up to the task.
If your genres are fast, dynamic action, a modern mirrorless system will deliver a higher hit rate in comparison with the 2000s DSLR. It's not that the DSLR is incapable of producing a quality, well-focused image. It's that the modern AF system is capable of producing many more usable options from a burst.
A Z9 on the hands of someone who knows what they're doing can deliver 16-18 images in a 20 frame burst that are usable. A D90 can deliver 2-3 frames from a 4-5 frame burst over the same time. A D500 can deliver 7-9 keepers from a 10 frame burst. A D5 can deliver 9-11 usable images in a 12-frame burst.
The most significant limitation of DSLRs in comparison with modern mirrorless bodies isn't autofocus performance. It's the burst rate. The Nikon D6, their last flagship DSLR, is limited to a 14 fps burst rate. Several Nikon Z bodies can shoot at 120 fps when recording JPEGs.
The Z6 III and Z lenses have day over night AF performance compared to 20 year old technology.
Would you expect the performance of a current laptop to be no improvement over a 20 year old laptop?
The laptop comparison nicely illustrates the flawed analysis of the OP and others in this thread. Twenty years ago, that laptop was capable of running apps, surfing the web, and performing any task the vast majority of laptop users did. People weren't buying and then mothballing new laptops because the devices available in 20 years would be so much better.
If all one uses a laptop for is perusing DPR threads, using apps like Word, Excel, and PowerPoint, sending email, and similar tasks, an older laptop is still capable. It may not be compatible with the latest OS but it has the necessary computing power to be productive.
If one uses a laptop to play Borderlands, Cyberpunk, or Red Dead Redemption; or to edit and watch 8K video, then you absolutely want a more capable, modern system. It's not that older tech wasn't incredible capable, usable, and reliable in its day. It's that new tech is capable of dying things older tech couldn't.
A modern mirrorless camera can shoot at 120 fps compared to 16, 14, 12 or fewer fps for a top shelf DSLR. That's the real game-changer.