No matter what, publishing 3.5 material will eventually be a slow ship to oblivion. Plenty of people are satisfied with 3.5 and won't switch over to 4.0, regardless of how good it is. But plenty more will switch over to 4.0. But the number of people playing 3.5 will only get smaller, not larger.
So what's a 3rd party manufacturer to do? 1) Take the bull by its horns. Those that will still be playing 3.5 will still need adventurers, monsters, settings, etc. WotC not putting out that material for it anymore means there will be a void that needs to be filled. A manufacturer that steps up and publishes a well thoughtout, innovative, and large scale setting before the mass teasers of 4.0 begin can help to reenergize the 3.5 base. Having this setting in stores (not just PDF/POD) will go a long way to propel a small company into the big time and establish it as one of the new major players. Being daring is key to maximizing the life of this option. The base will quickly dry up of all that is put out is $2 single class supplements. (Giants in the Playground, The Vorlon and The Shadows are leaving, now the Humans and the Minbari and others must step up.)
Note: This option will also require tough choices on the part of the companies. Companies that keep putting out products that are of limited success will not last long. White Wolf use to put out a number of small supplements for their Exalted line back in 1st ed called Caste Book (similar to race books). They didn't sell well at all. They were to narrowly focused and didn't contain enough eye catching material to sustain them. WW kept putting out the Caste Books despite losing money on it (perhaps out of tradition, perhaps out of completeness, I don't know), but when they made money on a larger supplement that covered many different topics (Outcastes, regional setting supplements), they changed their model for 2nd ed Exalted and now all their supplements sell well.
2) Jump ship. Having your own system (True20, Quick20) or writing for a different OGL system (OSRIC, Mongoose Runequest) will give a company a new market. But they will be behind those that are more established companies and will be playing catch up to them. Some companies will cooperate (Green Ronin-Ronin Arts, Necromancer-Paizo); others will licence game/systems from other companies (Mongoose). But this option requires them to live off another and are subject to external events outside of their control. (i.e. everyone remember what happened to WEG after they lost Star Wars? And they have their own system.)
3) Grim Reaper. Invaribly, some companies will not survive the changing landscape. Those that cannot adapt well to the new environment (or don't make the hard choices quickly enough) won't continue on. They'll be bought by others for pennies on the dollar after much debt has arisen. Sad, but it is true.