The notion that OSRIC's legality must be notably questionable until there's a lawsuit over it (which could be never) seems to me bizarre -- especially as I see no reason it should not apply likewise to every OGL product.
The difference between OSRIC and every other OGL product is simple. It has nothing to do with the fact that OSRIC is released using the OGL.
3e
was itself released under the OGL. And, with the System Reference Document (SRD), WotC made explicitly clear what content was open. Everyone who follows on there has a clear reference for what is okay.
1e was released under normal copyright. No other license applies until WotC says so. There is no clearly defined and accepted 1e SRD. The people who have created OSRIC have attempted to suck out the system logic (which cannot be covered by copyright), and restate it without use of WotC IP that is covered by copyright.
The question of OSRIC's legality is the question of whether they did scrub out all the WotC IP.
Meanwhile, WotC apparently can withdraw at whim the GSL.
Yes, with a six-month sell-off period. Given that I'm told most product make the bulk of their profits within three months of publication, this seems reasonable.
This is not comparable to the risk of using OSRIC - as I understand copyright law, if you sell something in violation, the copyright holder can not only cease your publication, but also go after a chunk of the money you made off of infringing.
Mind you, it wouldn't make sense for WotC to do that unless you made enough profits to make it economically advantageous to do so. Publishers who figure OSRIC and their own products will remain small-change operations have little to fear.
Honestly, I brought up the legality only for sake of completeness. I think the real barrier to OSRIC getting into wide distribution is that is it a niche of a niche product. Distributors and retailers got bitten kind of badly by having a mass of 3.x material that didn't sell. They've scaled back to things they now have more reason to think will sell - proven publishers working on editions that have a proven share of the market.
OSRIC is neither. That's a big speedbump.