It's business not blind fandom. Disney is counting on retaining the customers who don't feel that this policy added to their experience in any meaningful way. Why continue losing money on something that isn't considered valuable enough by your customers? With the assumption that they will retain many/most customers, they hope to earn additional money. If a person drinks 2 drinks per day and used to bring their own rum, they would save $20 per day. Now, they purchase those drinks from Disney and the line earns and additional $20/day (because let's face it, drinks are almost all profit.) If even 1000 people per cruise are spending $20/day, that is an additional $20,000 per day. Will it have an impact on the individual cruiser who only drinks 1-2 drinks a day, not really. But for
DCL, it can make a big difference. Perhaps those people even choose to buy drinks rather than an additional photo at Shutters which doesn't change the customer's overall expenditure but does change the company's bottom line (higher profit margin on drinks.)
There is also the reality that some will feel that BYOB is crucial to their cruise experience, and DCL will lose them as customers. My guess is that they have done sufficient research to suggest that they have enough buy in from the customers who want "family friendly" to drop this perk and have minimal attrition.