One more nod of agreement with the above observations.
Such a miniaturized spectrum analyzer would be too small for any kind of detailed analysis. I run SPAN or MMultiAnalyzer as large as will fit on my screen.
There are other benefits to a standard plugin, such as the ability to insert the analyzer at any point in the signal chain or have multiple instances on one track. Plus I'd just be reluctant to go back to the days when spectrum analyzers merely painted a graph; nowadays they often do much more.
To OP I'd say: it's not a bad idea, nobody's mocking it. But there are already better ways to analyze your master bus.