Well I only go to NDK, so honestly can't speak for other cons. And sadly I missed the panel on struggling conventions at NDK this year...really wanted to go and forgot or I think we had left by then...whatever anyway!
It wouldn't surprise me that some conventions are cutting back on guests. I'm sure they take a lot of money because the have to be flown in, then there's the room and any other accomodating needs they need. The cons probably pay to feed them and all as well, plus they have to provide security, as well as other staff like assistants and all that. Bet it adds up. NDK staff would probably have a more accurate assessment of what goes into guests. Oh and forgot, just the general fee of having them attend as well, can't forget that.
The thing is two, sometimes the actors are just busy. The con may enquire to a couple dozen actors and maybe only get replies from a few, and then still some of those may end up having to drop out or would like to attend but are already busy. So in the end the con only has a few guests.
This year we had so many because NDK wanted a big bunch for the anniversary. Honestly I hope we have a smaller group next year because it'll make it easier to try and get autographs when you don't have five people pretty much for every autograph session.
I don't honestly think it's anything to be scared about, or even worried about much. I think conventions will go on, and I'm sure there will always be guests attending. May be only a couple, may be more it all depends on who the staff of the conventions try and contact and who of those they contact are available and want to attend.