It’s not about the explosive growth (or not) of iMessage. It’s a matter of fact about the legal foothold that Apple now holds. That won’t be dislodged anytime soon. Whether or not Apple can get any market growth moving forward, now the EU will have to re-file any efforts to this ruling to them in the future should they try. That is a big deal. And nothing anyone in trying in the EU will move forward anything near the weight this attempt did.

What legal foothold, exactly? The conclusion that their messaging service isn’t important enough to negatively affect the market?
Whether or not Apple can get any market growth moving forward
If they don’t grow, the EU won’t need to do anything, because the status quo remains.
And nothing anyone in trying in the EU will move forward anything near the weight this attempt did.
The commission that did the initial market research is legally obligated to continuously monitor markets. This isn’t a one-off thing, nor was it an “attempt” at anything. If the EU wanted iMessage to be opened up, they would’ve written the law better.
The DMA is a law to protect against de-facto gatekeepers, and iMessage just isn’t important enough to be called a gatekeeper.
It’s not about the explosive growth (or not) of iMessage. It’s a matter of fact about the legal foothold that Apple now holds. That won’t be dislodged anytime soon. Whether or not Apple can get any market growth moving forward, now the EU will have to re-file any efforts to this ruling to them in the future should they try. That is a big deal. And nothing anyone in trying in the EU will move forward anything near the weight this attempt did. 
What legal foothold, exactly? The conclusion that their messaging service isn’t important enough to negatively affect the market?
If they don’t grow, the EU won’t need to do anything, because the status quo remains.
The commission that did the initial market research is legally obligated to continuously monitor markets. This isn’t a one-off thing, nor was it an “attempt” at anything. If the EU wanted iMessage to be opened up, they would’ve written the law better.
The DMA is a law to protect against de-facto gatekeepers, and iMessage just isn’t important enough to be called a gatekeeper.