There is a feature request here to create a more robust category system: Formal Race Category System - #2 by Christopher_Grote_B
The idea would be to follow the practices that exist in pretty much every country in real life: which aren’t based on your power rating at all, but (surprise!) how well you actually do in races.
You earn points based on certain placings in races, and once you have a certain number of points you move to a certain category. You can’t compete in lower categories once you’ve qualified for a higher category (unless it’s an “all-categories” / multi-category race).
Could people decide they want to always race in low categories? Sure, but they couldn’t win (or they’d get the points to qualify out of the lower category). So they’d be riding to prove that they can’t win a race, and can’t move out of the lowest category, so that they can brag about that? Sounds pointless (pun intended 
), and regardless they wouldn’t be creating an issue for others that are actually at the top of that category because those others would be left to win (and gain the points to qualify for a higher category).
Meanwhile people like me (and I suspect many others) who even with a relatively high average power, but who still can’t win races, will be really pushing themselves to qualify out of the lowest category for some proper bragging rights. (And the people we lose to will be bumped up to higher categories because they’ve won the races and earned points to push them out of that lower category, so the field that we’re competing against is constantly changing so that we can be more competitive in our next race.)
Without this kind of system racing in general in Zwift seems kind of pointless to me? I mean, I can perhaps average 4w/kg as an FTP (putting me in category A), but I weigh 62kg – someone who weighs 90kg can be a category C rider (needs to average more than 2.75w/kg) and they can absolutely destroy me in a flat race (due to a higher overall wattage). Of course, on an Alpe du Zwift race I’d destroy them.
But this w/kg-based category system doesn’t seem to account for these differences – and it doesn’t really encourage people to move categories as it’s based purely on power output (significant changes take months of dedicated training) rather than race tactics, course-specific strategies, teamwork, etc – all the hallmarks of what racing is really about, surely?