How old is Maggie? That would be one element of any decision that I would make, as would the result of the MRI. If the syrinx is quite small, or is long and quite narrow, then I would probably not choose surgery, at least not now, especially if she is an older dog. If the syrinx is wide and especially if it is lopsided and it looks likely to progress, especially if she is younger, then I would be considering surgery.
There is no solid evidence that doing surgery younger improves the dogs chances. some neurologists think this possibly could be the case, but that is a long way from actually doing a study to confirm it.
The difficulty is that different neurologists think differently on this topic as well, so it is hard to get a hard and fast answer. There are some who were very gung ho for surgery, particularly in the US, and there are some who prefer to work with medications. But medications will not reverse the problem although there is some evidence that perhaps CSF inhibitors can slow or stop the development of a syrinx, especially when it is very small.
I have two dogs who have never done more than scratch mildly. One of them I have now had for four years and she has never progressed and her syrinx is quite small. The other is seven and also only scratches. For either of these dogs I would never have considered surgery, and I strongly disagree with the neurologists who say that as soon as you see there is a syrinx, you should do surgery because SM always progresses. It may “always” progress but as with any health condition progression is highly variable and I cannot imagine having put either of these dogs through a very invasive surgery that often has a quite long recovery and potential complications.
I also know of statistics from a study that show over half of dogs over age 5 who had no clinical signs at all scanned with SM. If the condition “always progresses” then for many many dogs it certainly does not progress to a point were it even causes symptoms that would ever warrant removing a piece of the dog's skull. That said, older dogs can always progress.
My own parents are medical people, and they would feel strongly that you don't do surgery until there is a good reason to do surgery, with any condition. with SM, there are many points at which it might be a good reason–but I do not think just the presence of the syrinx and a very mild symptom is the starting point for taking that decision right away.
I stress again that this is my own opinion but it is based on both my own experience with dogs that have not been put through surgery, and knowing many others that are older and either have SM or have not been diagnosed but I would say surely do, where I cannot imagine that surgery would have been a sensible option. The difficulty is–it's a guessing game. And of course there are reasons why people feel they would never do surgery, and those are perfectly valid too, as is the decision that surgery offers the best option and should be done right away. It is such a personal decision and choice.