Current LLMs are manifestly different from Cortana (🤢) because they are actually somewhat intelligent. Microsoft's copilot can do web search and perform basic tasks on the computer, and because of their exclusive contract with OpenAI they're gonna have access to more advanced versions of GPT which will be able to do more high level control and automation on the desktop. It will 100% be useful for users to have this available, and I expect even Linux desktops will eventually add local LLM support (once consumer compute and the tech matures). It is not just glorified auto complete, it is actually fairly correlated with outputs of real human language cognition.
The main issue for me is that they get all the data you input and mine it for better models without your explicit consent. This isn't an area where open source can catch up without significant capital in favor of it, so we have to hope Meta, Mistral and government funded projects give us what we need to have a competitor.