I don't buy Muse being on an upward trajectory though.
Eh. That really depends on which data you look at. I mean, if you use the Billboard 200 charts as a reference, you can actually see a very obvious upward trend: Origin of Symmetry peaked at #161, Absolution peaked at #107, Black Holes and Revelations peaked at #9, The Resistance peaked at #3, and The 2nd Law peaked at #2. This is hardly an indicator of quality, but it is certainly an indicator of popularity. Meanwhile, Madness set a Billboard record by topping the Alternative Songs chart for a total of 19 weeks.
To your point, I think most people who would call themselves Muse fans tend to prefer the earlier albums - not all, mind you, but most. I myself count Origin of Symmetry, Absolution, and Black Holes and Revelations among my favorite albums of all time, and I doubt Muse will ever top them for me, so I certainly prefer their earlier albums.
But I think more people are listening to Muse now than ever before. And with all that mainstream attention, they're still managing to maintain a fairly (but admittedly not entirely) positive reputation with their longtime fans.
I'd say there's a strong argument for an upward trend.