I expect them to improve because there are obvious, straightforward, practical improvements they can make already. As an obvious example, the NLP is currently limited in terms of the variability of ways to add things to your shopping list. Adding more variants of that would be easy.

The huge amounts of data they're gathering about what people actually use this for is indeed a huge advantage. With that kind of data, they can build a centralised dictionary of common patterns for asking just about anything. Because of Siri's practical orientation, there is no need to build a "generic NLP engine" (which is what you're, quite rightly, suggesting is slow to improve). They merely need to improve the effectiveness of the NLP engine they already have, by making it understand more common patterns. That's achievable through expert-system-like functionality that's existed for 30 years.

In terms of the voice recognition, that has been progressing steadily for 30 years too, and I imagine Siri will benefit from improvements to the technology base, as others will.