Maybe plateau is the proper word, so long as it doesn't imply summit. And it's not a boredom thing, I still look forward to brewing. I'm actually brewing right now. Its more of a plateau/rut as far as trying to learn new stuff, explore new styles, bla bla bla.
The brew day is fun and relaxing, the beer is good, I just don't care anymore about science and proving things.
Jim, I'm in much the same boat, but I think of it as "focused" rather than in a rut. I make the styles of beer that I like, and they're pretty darned good (says he, throwing his shoulder out while patting himself on the back). There are some styles (sours come to mind) that I enjoy occasionally, but I'm not very interested in brewing; those I drink at one of my local taprooms. I do enjoy IPAs and pales right now, plus some sort of Belgian now and then. I have some "standards" that are pretty much always on tap (because my wife, my friends and I like 'em). When I want to do something a bit different, I'm likely to try something like Denny's Rye IPA - not a big departure from my "usual", but I can be pretty sure I'll enjoy it. I love brewing and my beers, and that's what it's all about for me.