I am considering moving my personal account to iCloud now that you can set up a custom domain with it. But for most of my accounts, they're Google Workspace accounts paid for by various different employers. There are few if any viable alternatives for their use cases.
Plus, I've not seen anything that can beat the Gmail web interface. I'm not a big Google fan — especially in terms of data mining — but for business accounts, you just can't beat the power of Gmail searching and labeling. I'm happy to be proven wrong and switch to a better alternative, but that hasn't happened in the last decade plus I've been using Google Apps / Google Workspace / Gmail. As much as I like the idea of cutting off the data mine in theory, I also have to balance utility and ease-of-use for myself and my clients. Not being part of their dataset offers me no comfort if my clients and I are unable to complete or are significantly hindered from completing business processes and functions.