Two points here I think:
1) Yes it is possible to build maintainable triggers - I'm doing it right now, as a matter of fact. However, it's certainly not my first choice since it's (a) hard to debug when they cascade and b) performance is hard to keep under control as they proliferate. Just look at Oracle's older products (or heck, look at Apex) and the huge amounts of triggers firing for even the most simple of tasks. It's a weed that you have to control rigourously or it will grow out and suffocate your software. It is NOT a best practice to use triggers if you can avoid them, they're a last resort if all other options are off the table.
2) Centralizing the business rules has a lot of repercussions beyond the technical side of things. Look at BeInformed's products for that. With proper definition of business rules, a good business rule engine can generate most CRUD-code from scratch, dynamically populating the screens with the required fields. BeInformed's latest product even generated the workflow at runtime, all business rule based. It was much more advanced than reactive. Unfortunately they invested too much and they're now up for grabs as they went under. As I understand it, SAP and Microsoft are fighting over the remains. Which is a much better buy than Reactive.
I'm sure you can find an edge case where some platform can't access the centralized business rule repository, or needs an exception. Or it becomes inflexible and unwieldy. Those are generally signs of failing organizational processes, not technical issues.
That said, there's another point: why can't databases integrate with business rule engines? We're still stuck with constraints from the 60's, even Domain-Key constraints have to be programmed instead of declared. Databases could leap forward if they would deal properly with time, versioning, and business rules. Instead, we get slightly higher marks on the TP benchmarks. That's useless.