The battle of the virtual assistants could be over. This battle started on the smartphone but progressed, in 2017, to smart speakers and home automation. On smartphone, your virtual assistant could be activated by voice or touch, answer questions, read email, advise on the weather, provide directions, tell you about your appointments and much more.
The technology was based on the idea that consumers are no longer storing their data on their PC or on a memory card inside their mobile phone but in the cloud. In this instance for "cloud" substitute Google, Apple or Microsoft.
Apple's Siri was one of the first. It debuted on the iPhone 4S. Microsoft announced the most personal assistant on Windowsphone. The Microsoft entry was called Cortana - after the helper in it's Xbox games series Halo. Google launched Google Now followed, in 2016, by the Google Assistant.
All of these mobile tools could talk to you or you could ask queries. In effect replacing search. The other side of all this help was that your data was now in the cloud. Your location, preferences, transport choices, work place etc. Google, for example, could now target an advert for a special offer based on where you were each lunchtime. The vast data volumes created allowed all kinds of new opportunity for services to end up being useful for consumers but also useful for the cloud service providers.
Amazon had no mobile platform so it created the Amazon Echo. A digital speaker that, among other things, could let you buy stuff from Amazon just by talking to it.
The important features of a virtual assistant is that it's always available, it's everywhere, it learns new stuff and it's genuinely useful. It also needs to understand context. So if you ask about the weather in Paris and then ask how far "it" is then the virtual assistant needs to understand what the "it" is and you still mean Paris.
Microsoft decided in 2017 that it was now out of the mobile phone market. This relegated Cortana to the PC. Google and Apple retain their virtual assistants on mobile. Amazon has effectively created a new market for an assistant in the form of the smart speaker. Moreover, both Google and Amazon have allowed people to add functions to their devices as "skills".
No one is sure where this is going. What seems to be happening though is that Microsoft is falling behind for consumers. Absence from mobile, the smart speaker market, lack of context for Cortana and lack of availability in many markets is making it less useful. it may have a second life as a business service being a virtual assistant for office 365. However, in January 2018 the virtual assistants that have most consumer traction are Google Assistant and Amazon Alexa (Echo).
Cortana won't disappear because it's really just an interface to the Microsoft Bing Search engine. It's most likely future is in the enterprise rather than in the smartphone or smart speaker. Apple is an unknown quantity but it only seems to be interested in Siri as a phone helper for iPhone users.