As a developer myself that relies on many API's from other companies I think I can shed some light on what is going on with Siri.
When you write a piece of software like this that has to pull in information from other sources, for example with Siri it uses API's from Wolfram Alpha, Google, Wikipedia and various others.
What Apple does is write an interface to talk to these API's and they need to write in really specific transations from what you're asking and what Siri then needs to acquire from the service API's.
For example asking for a movie review vs asking for movie tickets means Siri needs to first identify you're talking about a movie then select the right service, Rotten Tomatos for Reviews and a Movie ticket service for tickets.
The problem with having so many API's added to Siri (and they probably are at or over 100 by now) is that these rely on other companies.
These companies don't just write a web API to access their data and then sit around for 36 months twiddling their thumbs. They are continually adding new features to the API's changing the API responses, making available new data or removing access to data.
You can't just write an interface for an API from say a movie ticket company and then leave it stagnant for years. You need to look at it again every 3 to 6 months to see what new features the API has, what old features have been depreciated and how the API responses have changed.
Apple hasn't been doing that maintenance work. The API's they're using are evolving and making things break, becoming incompatible with the interfaces Apple wrote to talk to these API's. And from what I've seen when they realise somethings broken they are using cludges to maintain compatability.
They need to rewrite some API interfaces in Siri to be 100% compatible instead they are dropping some features they used to have because they don't want to rewrite the interface to use the new API contact methods.
This happens all the time. I make use of about 7 API's currently and about 4 out of those will change every 6 months necessitating my interactions and about 2 out of those 7 will break compatibility completely every 12 months.
I believe Siri is becoming worse, not better because of a lack of maintenance by Apple on the API services. She is becoming less informed.