Changing, or not being able to change, the AppleID, is an indicator of who has the most power, the end user or the service provider. Google does allow you to have multiple IDs (email addresses) and to set one as a main ID. I can choose to change the ID associated with my Google Voice account, for example, and have, without losing the number.
Apple has NO provision for changing the AppleID. If you choose to create a new AppleID, none of the purchases made on another of your IDs can be transferred. Apple promised to fix this years ago and has done nothing. I read it as an sign of their inflexibility. Apple wants to control my tech environment. In a lot of ways this is good - there are many advantages to having control of both the hardware and software in terms of delivering performance and reliability, but it does lock you in. Apple hardware is more expensive, because there are no generics to choose from. Easier for them to support.
The OP's question, I thought, was about which email service is best, which is a different question than which email client is best. I like Outlook, it's a good client, but now they are moving it to the web unless you purchase a copy for your local machine, and this means purchase regularly, because they have gone to a subscription model for Office. I have no beef with Mail on the Mac, works fine for me. Gmail app is good too, for mobile and for webmail. Can't speak to iCloud's webmail, haven't used it.
I figure that Google has a good offering in terms of price (free) and storage (20 GB, I think) and they've been reliable about keeping it big and free, and they don't force me to use any of their other apps to get that free email. iCloud is free for the first few gigs, then you have to pay, and honestly I don't quite get iCloud. From what I can see, it's like Dropbox: data in iCloud gets replicated on all your devices, and it starts to use up the storage fast. I thought it was cool to have all my phone photos automatically backed up to iCloud, until the storage filled up. Can't delete things off iCloud because it will delete them off my phone, too. So I went and moved everything out of iCloud, copied it to another location, so I could free up space and not loose data. To me, this is just a hassle. I want a backup to be something I don't have to think about. Apple is starting to integrate more and more stuff to iCloud, and while I like the handoff idea I don't like not being really sure about how it works and how to manage it without thinking, because I know I'll goof it up if I have to manage it manually.
That's my only concern about iCloud mail ... what will happen in the future ... as far as availability and price, and what data will I lose if I have to change an iCloud email ID, the way I had to change my AppleID? Maybe this is all moot, as it seems that email is fading away to be replaced by something better, who knows what ...
