Take what minniev says, and you know you've hit a plateau when all the above are in play and your photographs all "look" the SAME! It means you're "seeing" the same.

GET OVER IT! See different: high, low, long, wide, vertical, landscape, near, far, morning, night, winter, rain snow and make them LOOK different and you're climbing forward again! The new lens won't hurt, but it's a stimulant not a fix.

I found that simple 50 exposure self assignments get me seeing creatively, again. OR the same subject, different times and lighting. OR the same subject with an emphasis of relating the parts/shapes to the whole. For example, find a motorcycle and show all the circles, hubs, wheels, sprockets, chain fasteners, dials, gas cap, and on and on. You'll fill up a BIG card. And the next time you see a bike, you'll find a hundred more. Then see the harmony and trends. Render them in B&W. Enjoy the tank paint job colors. What's not to like. and you can't do that sitting 20 feet away. Oh, and don't forget all the features and distortion effects of CHROME! That's a shoot all by itself.

I don't find any plateaus, just mountains, because:

(1)My camera knows more tricks than I am ever likely to learn

(2)My software can make one photo take on thousands of different looks

(3) My adorable grandchildren are growing up so fast, changing each and every day

(4)This big old world is so beautiful and complex, there is no way to run out of interesting things to take photos of.

(5)Even in my humble and non-photogenic hometown, there are no two sunsets (or sunrises) that are exactly alike.

I think most artists, not just photographers, hit a brick wall at sometime, but thats not to say they have reached a plato. Occasionally a really great picture comes along which make you think where do I go from here, or you are fed up with what you are photographing, and it may seem a bit flat for a while, but there is always another great light, portrait, special moment or the like waiting to be captured, and there is never a time when there is nothing more to learn.
Maybe if someone feels that have reached a point where they cant see a future in photography or anything else for that matter, it is just that, and it maybe time to look for another interest of pastime.
One thing I do know, I don't know anyone who has taken the perfect picture, and thats what drives most people on.