I definitely agree newer systems especially with electronically scanned radar systems, that tracking multiple targets is certainly a capability. I also believe some weapons would be intercepted and destroyed in flight, but certainly not all. The thing about intercepting small targets, especially ones that are moving quickly is it takes a lot of lead time, you have to be prepared and know where to look for the threat. For example the US Aegis destroyers (we have 33 of them) are a platform designed to intercept and destroy high flying fast missiles. While the Aegis has proven the ability the detect, intercept, and destroy targets such as missiles and the like, it has a far from 100% success rate. It also has an AESA radar system and in fact we’ve failed as early as this year at intercepting a ICBM during a test. As you mentioned in your post, speed/distance, and other parameters affect the engagement zone, so the SAMs would have the highest pK against a launch as the target gets closer to the ground, but if tracking a large missile that is a couple times bigger than a telephone poll with Aegis ship based system results in misses (29 killed out 37 tests total about 78% which matches with your BUK-M2), then the little SAM operators on the ground are definitely going to miss some SDB/HARM as well. It is reality vs theory on paper.
I think someone brought up in this thread earlier that there is a perception that the SAM operator is the hunter and the fighter jet is the prey, but I do agree with him it is the other way around. I’d hate to be a SAM operator with a qualified SEAD/DEAD trained flight lead and wingman coming to hunt me down, no matter what system I have at my disposal. Now one thing I do see going forward is the capability to engage small RCS targets increasing, hell they’ll probably have to make stealth versions of the HARM/SDB in order to reduce its RCS even further if it becomes an issue. Low vis coatings, change the shape of the weapon a little and bam its smaller than a mosquito and bites like one too (given that we can make an F-22 appear to be the size of a bird :p).