I'm not necessarily saying the discs will break. I am, however, saying that different disc models will be made to different thickness, and not all discs will be of an even thickness. There may be issues with discs warping at high speeds/certain RPMs, and some discs just don't have a great balance. For these discs, if they could be read at 24x, I'd imagine that they'd make a great deal of noise [something most consumers don't look for], or would require a new drive pick-up mechanism [designers would have to make a new pick-up mechanism to compensate for poor balance].
Still other discs are physically crippled, but in a more minuscule way. For example, a B-grade 16x rated disc may only be written well at a lower speed, like 12x or 8x or even lower sometimes, and the physical problems with these low-quality media will also require a lower read speed. [The physical problems may cause tracking errors, focus errors, etc]. I'd imagine that very few discs have been machined to a level of quality at which the drives can keep a proper tracking and focus at the 24x RPM to the outer edge of the disc, even if the disc had been written to the best possible quality.
There may still be the issue of a poorly designed/machined/bonded disc becoming unreliable at the 24x RPM and breaking, but I don't see that as being too much of an issue.
If 24x reading was supported, it would likely be on a per-disc model basis, probably only supporting the same media as is supported for 24x writing.
I could see 18x reading being incorporated, but the promise of reading discs at 24 speed seems about as useless as writing discs at 24 speed; too many discs would just fail to be read stably at that speed.