I think the issue is that all of those Google services are really what makes Android Android. An Android phone lacking integration with Gmail, Google Calendar, etc, without something else to make it compelling (like the Kindle Fire), would be a pretty tough sell I think.
That's true, but if say, Microsoft, wanted to, they could clearly create replacements for those services. Now, it's arguable whether an Android phone with Hotmail + Bing Maps integration is even an improvement, but at least it's possible.
You could say the same for GNU. What is Linux without a DE? And a fancy DE like GNOME Shell pulls in a lot of dependencies, the latest furore in that area is GNOME&systemd.
Difference between systemd and GMS is, developers aren't making money off of systemd being in their control, Google relies almost wholly on it for Android profits.
Android is a fully functional Linux based operating system. To expect GMS to just be handed out like it was a keg party flier without considering the actual economic reality of running Google is ridiculous.
OEMs like Samsung weren't fooled back in ~2004 to think they'd get an amazing OS for their phones for free, and the deal Google is offering is beyond amazing compared to how other (tech) firms do business. imo.
What it comes down to is really "what do Android buyers expect from an Android device?"
If the consumers expect that they will get email, cloud storage, calendars, and maps then it would likely be fine for Microsoft to create similar integrations. If consumers expect that they will get Gmail, Google Drive, Google Calendars, and Google Maps, then they will likely be disappointed.
Remember there are billions of people who haven't already been locked into Google services. They don't think "I need access to my years and gigabytes worth of Gmail." They think "I want email. This 'outlook.com' thing that came with my [hypothetical MS Android] phone will work fine"
