Change in itself isn't a good thing, I agree. Here's where I'm coming from:
I've never like the jam-band genre. Actually I hate it. So of course if a band like The Dead stay as consistent as they have, I will continually hate their music. If you like their music, I can see how this is a good thing, but...
Exploring musically is an admirable trait. I loved The Mars Volta's old stuff, but they got too convoluted and now I'm not a big fan, however I respect them for evolving because if they kept re-making Deloused in the Comatorium, their music would've lost the zest regardless, only they would lose my respect for getting stuck in a rut.
Pretty much everything Roger Waters wrote for Pink Floyd I hate and consider it to be the worst music the band has ever put out (The Wall's finer parts were 100% Gilmour, despite the rumors). However Rogers's orchestral arrangements bled into Gilmour's solo works later down the line and him and Wright produced some of the best music of the 2000's. Even bad change can lead to good.
I'm not saying a band's failure to evolve makes their music at any given time boring and stagnant (The Dead just happen to be a band that I find to be those things for other reasons), but over time I get sick of even a good thing.