I think Siri In A Can will eventually come to market but I wonder what the bullet points will be to sell one. Siri already integrates into HomeKit and supports many more languages than the Alexa platform but that only makes two and both of them can easily be dismissed with Amazon's recent push into the home automation space and a simple (or complex) update for more language support.
Apple has a huge advantage here to see how and what the market is reacting to. They can take their time, see what the market really wants, and make adjustments to their product. People tend forget that Apple wasn't the first company to bring a smartphone or tablet to the market -- they were pretty late in retrospect -- but the products they did introduce were refined to solve the problems people needed solved the most. You certainly don't have to be first to market to win the technology game.