That depends on who you talk too. Overall they are declining. And I would say that considerably is not necessarily and understatement. The market has been in a period of adaptation for roughly the last 7 or 8 years. I have not been in the market and assisting that long. But . . . since things are transitioning to digital. The responsibilities have shifted and alot of skill sets are either becoming extinct or no longer necessary.
I started in the photo market. And I have worked professionally under some of the top names in the country. But . . . I really doubt I would be able to live as comfortably . . . though it is not that comfortable by a long shot if I did not have other skills to market and boost my day rate. Such as Capture One . . . teching . . . retouching . . . handling RAW workflow. Doing entry level photography, design and web work.
The straight out Grip positions . . . film loading, taking to the lab, etc etc etc are just not around. And digital has made the market more competitive and has driven the price of the entire process down. Productions are smaller now than they were even 3 years ago.
Just my .02. If your going to get into the assistant market. And that is all you can market. I reccomend you try and get in with a studio that can take and carry you full time . . . other wise. Really see what other production responsibilities you can market . . . even if it is location scouting . . . to supplement your income . . .
And always know what to charge . . .
Best of Luck