I think something to consider would be the manufacturing costs of producing quality glass. While the laws of physics may not change and lens technology may be slow to evolve, it's possible a new technique for creating quality glass cheaply may be created. Example: Ford came up with the assembly line for cars, making them considerably cheaper.
Or perhaps SLRs will become so mainstream that they will start producing lens in a mass enough quantity to reduce cost per lens. Or maybe Canon will introduce yet another new mount, making all EF lens obsolete.
Don't forget about technologies like Diffractive Optics. It makes lens shorter. I don't know the specifics, but if a technology like DO make lens require less glass (I'm not saying it does, since I'm not that familiar with DO) it would make sense that it would make lens cost less (assuming the new technology is reasonably cost-effective).
And once again, saying this without any concrete knowledge of lens work, why don't they just open the aperture opening larger for lens? In some cases I understand it's a physical size limitation, but in other cases I hear it might be because the quality of the image suffers greatly. But with improved optics, a wider aperture might be more acceptable.
What if they discover a new material to make lens out of? It doesn't always have to be glass. There's fluorite as we all know. I hear they even have plastic lens, which may sound low quality, but who knows 10 years from now.
And I'd have to agree with vic6string. If technology continues to advance by creating sensors that have more megapixels, better dynamic range, and high ISOs, and creating lens that are high quality, better resolution, etc, then there may not be any need for SLRs. It's probably going to be true that larger lens and larger sensors will always have better quality and less noise, but there's a saturation point at which people don't care or simply cannot tell the difference, in which case 99% of the time the point and shoot will do just fine.
An example of this is video. At a certain FPS our eyes simply cannot tell the difference between faster frame rates. However, there may be specialized needs for high speed video cameras to catch things in slow motion. That's an example of that 1% case that you would need something more.
The only real limitation I can see is that lightwaves are of a certain length and past a certain point we have to deal with the effects of diffraction. But hey, who knows, maybe we'll create a lens that decreases the length of a lightwave to exceed even that limitation.