Own AMIs? Simply cost. No other reason, although we're evaluating it again, so we'll see.

We actually have several "massive" CDK projects now, depending on what metric you use for determining size. Our largest CDK app has more than 60 stacks, but with a cellular architecture that's artifially inflating the numbers a bit (n unique stacks against k AWS accounts where k > n but for n > 20, < 100) Maybe the speed at which we change persistent layers (99% additive) will slow down someday, but when you maintain a large number of services (>14) with constantly changing external contracts, it probably won't; it hasn't the last 6 years, it only gets faster.