There’s a difference been decoding voice requests and being able to answer any questions you may have; you wouldn’t expect Google web search or Wikipedia to work in offline mode, so why do you expect it to work for Siri?
I expect that it will one day soon work offline. Likely not a full web search.
By 202X, while offline my device should have:
1. Enough storage for voice to text models, translation models, local Maps data auto-downloaded when I have reception (including restaurants, reviews, image thumbnails, etc), Wikipedia, wallets (car keys, credit cards, concert/movie tickets, email, daily news, music/podcasts/YouTube that I’ve subscribed to. All of this should be automatically synced when I have reception (podcasts and music already do es this).
2. Enough compute to translate voice to text (and between languages), set a simple timer/alarm, unlock my car, search through Wikipedia, search for restaurants, directions between points on the map (without using traffic data), open an App or play downloaded music/podcasts/YouTube (including the option to chrome-cast without wifi - using BlueTooth (or similar with high bandwidth), NFC/UWB for simple pairing).
3. Auto-upgrade to use the network when it’s available, like use current traffic data for directions, hi-res images/audio/video.
Bonus: When offline, Apps like Spotify, Gmail, etc should also run some local search options for downloaded data.
Bonus: When offline/online, restaurants should have NFC/UWB servers for their menu. My iPhone should just “download” the menu from the POS system. No need for “signal” or wifi.
Bonus 1: you can search your offline library in spotify. Bonus 2: this used to be common back in the 00s but it was generally decided to be mostly spam. Bonus: local search for downloaded spotify stuff works without a connection.