Hey Groundhog,
The thing with diet soda is that it decreases the intensity and your exposure time to acid attack vs. regular sucrose-sweetened soda.
Sucrose is the stuff that will stick to teeth and allow colonization of Strep Mutans (via synthesis of sticky glucan matrix). Because it sticks to your teeth long after you finished drinking, sucrose-sweetened drinks will expose your teeth to acid attack for a long time.
This stuff is absent from diet soda, so whatever acid attack from the diet soda (carbonic acid) is transient and pretty minor in comparison, and gets diluted quickly by normal salivation.
The Zinc Oxide Phosphate and Glass Ionomer cements used to lute crowns onto prepped teeth actually dissolve over time in natural saliva whether or not you drink diet soda... Can't be helped, unfortunately.
So don't worry... Enjoy a diet soda! I'd much prefer my patients to drink diet soda than stuff like coffee, regular soda, or orange juice... Unless you suffer from phenylketonuria of course... 😀
The worst drinks that will compromise crowns would be sugared drinks and VERY acidic drinks, like orange juice (pH 3.5). If you drink OJ like three times a day every day, you WILL get acid-mediated erosion on your teeth, similar in appearance to people who suffer from GERD and anorexia.
HTH!