The companies I see continuing on will be those who have already broken from the pack. They might do some 4e stuff, but for the most part will probably keep doing what they have been doing. Companies like Green Ronin, Mongoose, and Fantasy Flight already have 3.x related independent products holding up on their own and continue that way.
Don't forget players who will play more than one game system. I've bought enough variety stuff over the years to keep a few manufacturers in business for a couple of years at least. Some will be willing to continue playing 3.x while moving on to 4.0.
The fact we don't know about the guessed statistics mentioned is out of that 50% or even 5%, how much of that will be pre-existing market already. If they aren't losing their market then there will be nothing to gain by moving to 4.0. Also some companies have developed a good enough reputation that some fans will buy the product anyway just converting it over. Another defining issue will be the new GSL, or whatever acronym WotC finally settles on, and if it will allow companies to still work with the OGL or not. If it does then I see companies like GR modeling Freeport stuff to work with both products if only posting conversion information on their website. Chris Pramas has already mentioned to some extent that this element will be one of the deciding factors for them. How much or to what extent we'll have to wait and see once they decide.
The allowance of the OGL will be an important aspect also. It has been both a blessing and a curse to the industry as we've seen. We've all seen how it has raised companies out of the ranks while crushing others. WotC needs as many people on board 4e as possible because of that. A company, because of the OGL, actually has the potential to destroy D&D if you really think about it. All it takes is for someone to come out with a game concept based on the OGL that everyone likes more than D&D and it would be game over. Think WoW when it emerged compared to EQ, most figured it would do well, but I don't think anyone really expected it to do as well as it has, coming to dominate the MMO industry. Both are online games, both play similar for the most part, and so there is no reason they shouldn't be running neck and neck other than people like playing WoW more than playing EQ or any other MMO for that matter. The more companies WotC gets onboard with 4e though, the less likely people will remember the OGL down the road, and the less likely something like this will happen. However if WotC denies it outright the decision will probably hurt them as everyone I mentioned rely on their dominate products, which all depend upon the OGL, and having to shutdown their main cash flow will be a big turn off.
I'm looking forward to 4e, but it has to live up to what has been promised and everyone has got to get on board if you look at it from a business sense thanks to the OGL. Small companies eking a living off the OGL and 3.x related stuff isn't the worry and they will thrive, thanks to those few who remain behind, without any problems. The whole thing reminds me of a Greek tragedy in how the hero does something he thinks is helpful, but in the end it comes back to bite him. I am interested in seeing how everything turns out come this time two years from now, finding out who lives or dies because of 4e.
Please realize the companies I've mentioned have found niche markets for themselves and are used as examples. Fantasy Flight Games have Midnight covering dark epic fantasy, Green Ronin has Mutants & Masterminds covering super heroes and True20 which has grown into its own game system but still based on 3.x for the most part, and Mongoose has Conan covering bloody epic fantasy. I almost forgot Sword & Sorcery has the World of Warcraft RPG put out through White Wolf.