More than the fines is the ability to impose changes on their business practices. Imagine if Google could no longer require OEMs to bundle various Google products in order to get the Play store.
Indeed, early on in Android's life, Google was much more hands-off with the OEMs, letting a thousand flowers bloom... we got fragmentation and subpar devices, developers and consumers complained, and Google set forth on tightening the reins and exerting more control such that when people buy an "Android" phone, it means a particular experience. (And of course there's nothing stopping OEMs from shipping AOSP-based devices and just not calling them Android - as in fact a number have done).
The issue I have is that if you're a non-techy user and you get accustomed the "The Samsung Way", then try to switch to a different brand device, it can be very disorienting. They practically feel like different OSes in some cases.
In addition, the current Nexus devices (5/6) are pieces of crap compared to the halo phones from the big OEMs.
But I do agree, that OEM skins do improve some lacking features of the stock Android, they also bring in tons of dark patterns and just terrible UX design with them - TouchWiz (Lollipop) for example removes all timed audio profile functionality, brings confusing duplicated apps for pretty much all standard applications (Play Store, Google Fit, Calendar, Mail, Keep, etc.) with their own separate accounts, hides camera functionality in public API and several other problems. Not to mention a completely different design of the OS from the whole Material ecosystem.
And thats a tip of the iceberg considering the times of Android 2.3/4.0 when Google CTS tests weren't so strict and developing anything worthwhile was practically impossible since OEMs kept overwriting even default integrated themes - stuff like setting black text to be white, removing fonts and other fun stuff we had to deal with.
Samsung phones have been degrading in quality as well - screens have become more fragile, Samsung has focused on copying Apple in many ways, and probably most damning, the market has responded negatively to these more recent changes.
Give me a more spartan device like the Nexus 5 over that crap any day. Google had it right, but unfortunately has been blocked from bringing that experience over wholesale.
But without Motorola in the picture, it's an easy argument to make that if the EU wants to encourage a competitive handset market (Android) over a monolithic one (Apple), the core product feature set has to be centrally controlled. Too much fragmentation of the platform would lead to the emergence of a dominant vendor (likely Samsung) and actually decrease competition as their competitors will no longer be at sufficient scale to support a differentiated product in both hardware AND software.
The thing is, the Google Play Store is more than just an app store, it's an entire ecosystem of apps and libraries that are guaranteed to be there and synchronized across the device. If you make Google services optional, then developers can't rely on their presence and would have to develop against multiple APIs. You'd get into a situation where an app would work on one manufacturer's device and not on another, and the platform would likely crumble over time as the user experience would pale in comparison to the tightly integrated iPhone.
Developers having to do real work is irrelevant, because developers are not the goal of antitrust law. Antitrust law is about protecting consumers, and if developers have to do more work because of it, so be it. If the EU's new limits on Google after this case causes their infrastructure to collapse or become unfeasible, that's Google's problem to solve. These are technical issues, not legal ones, and have no place in this discussion.
Developers having to do work is absolutely a competitive issue: it's erecting a barrier to entry through regulation because it drives up the cost of developing an Android app. For many developers, it would likely drive the cost beyond the point of feasibility.
And the market is likely at the point where even if the restrictions were relaxed, companies wouldn't take advantage of them because the market has already settled on Google and Apple. But if they were relaxed, it would likely be the end of Google's support of AOSP, which would further hamper the ability of OEMs to develop their own platforms / ecosystems.
Most other OEMs would take longer to set up their own apps, but they'd all follow suit.
And they absolutely compete against each other. They just don't do it in public because there are more effective ways of competing.