I'm not sure I understand the question. Assuming that one wants to make a 3.5 compatible product -- or is currently still selling 3.5 compatible prooducts based on the OGL -- the uncertainty of the OGL remains. The point of putting the 5.1 SRD into CC was to eliminate that uncertainty, no? So I am not sure why it would be different for the 3.5 SRD.
Here's the thing about that: the "uncertainty" around the OGL comes from not being sure whether or not WotC could actually live up to its threat of revoking the OGL. Even though most people (including most of the lawyers who chimed in on the issue, here and elsewhere) were
pretty sure they couldn't actually follow through on their threat, no one was 100% sure about that.
More importantly, no one wanted to be the guinea pig who went bankrupt trying to prove that in court.
But what most people seem to have overlooked is that this same scenario—where WotC issues a legal threat that
seems dubious to most people, but no one is completely certain, and nobody wants to be the one to try to put the lie to it—is just as true for the CC as it is for the OGL.
True, WotC doesn't own the Creative Commons like they do the Open Game License, but that's a distinction without a difference where making a probably-bunk-but-nobody-is-totally-certain threat is concerned. If WotC were to issue a statement saying "Because the
text of the CC-BY-4.0 doesn't say that a creator cannot 'withdraw' their work, we're removing the 5.1 SRD from the Creative Commons as of [this date]. Anyone who continues to use it after the date given will be prosecuted for civil and criminal penalties to the fullest extent of the law," most people would snort and say that statement was completely bunk, and they're
probably be right...but no one would be entirely sure. And I suspect no one would want to go to court to try and prove WotC wrong either.
(I'll point out that there's a lot more case law surrounding the CC than there is for the OGL, but the thing to remember there is that you have to already be engaged in litigation to make use of that. If WotC sues you for continuing to use the 5.1 SRD under the CC in the above scenario, then unless you can get a declaration of summary judgment dismissing their suit, you'll still be in for a long and expensive discovery process...and that presumes that all of the case law is on your side anyway.)
All of which is to say that no license can erase
fear, uncertainty, and doubt, even if it's intended to do exactly that. When the threat that's made is predicated on bogus legal posturing from a multinational corporation, the specifics of the license that's being threatened are relatively minor details.
The CC isn't any safer than the OGL, in that regard; it just looks like it is. So there's no real reason not to keep using the OGL rather than waiting for the CC to have the same SRDs published under it.