It’s possible that one can learn foreign languages just by talking to AI robots on one’s phone, but it probably isn’t a method where one would gain native fluency.
Firstly, you would have a hard time grasping the accents of people who actually speak your target language as a native language. Let’s take Mandarin as an example. Even though it is the language of instruction in most Chinese-medium schools around the world, it doesn’t mean most people actually speak like Siri, who is programmed to speak in “Standard Mandarin Chinese”, which is based on Beijing dialect of Mandarin. In areas like Guangdong and Fujian, many people do speak in an accent heavily influenced by their dialects, which are Cantonese and Hokkien respectively. This is in fact non-standard, yet the irony is that many people are more fluent in Mandarin than their dialectal mother tongues, which almost results in a separate standard which most natives eventually conform to. This effect can be seen most saliently in Malaysia and Singapore, where many Chinese-educated locals have to maintain a standard Beijing Mandarin accent when speaking in formal events, while they use their normal dialect-influenced accents in everyday speech. Malaysian-accented Mandarin would most likely be very alien to someone who has only ever heard of the Beijing standard.
Secondly, many nuances in speech wouldn’t be captured by Siri, and you wouldn’t even be aware of them. In Spanish, there is a very obvious element of word linking and elision in speech, which makes the language seem extremely fast to untrained ears. I have tried Siri’s Spanish voice and it has not been able to reproduce the elision effect I get from, say announcers on Spanish radio stations. This would be disadvantageous to learners, since you stagnate at a textbook level understanding of the pronunciation, which is arguably the most important aspect of being able to effectively communicate with speakers of the language.
Finally, many cultural memes and references would be obscured from you if you learn from Siri, since it probably isn’t smart enough to recommend those to you. A few years back there was this Cantonese meme on a Hong Kong student reading out a famous poem in an exaggerated manner, which rapidly spread throughout the Chinese speaking world. However, Siri probably wouldn’t have been part of the frontier of such action, since the spreading was done through popular Chinese social media like Weibo and WeChat.
What’s the point of learning to speak, when you do not learn to communicate?