It's already happening dude. Lots of the dope on the street is cut with rc benzos. Not to mention etazene drugs and xylazine. That's why so many people are dying. It's a toxic drug supply. This article is a couple years old but yeah the heroin is testing positive for benzos. I think I got a batch once myself. One of my last batches before I quit regular dope use 3 years ago.
Yeah xylazine is common, but in practice, based on thousands of analytical results from around the country, it appears to be extremely rare to find benzodiazepines of any sort in street fentanyl/heroin:
That sample is large enough in size and well enough distributed to get a pretty clear picture of how rare it is.
But if benzos were as common as xylazine, it would be some fucked up shit, creating binary addictions to both fentanyl and benzodiazepines. On the flip side, if benzos were as common of an adulterant as xylazine, the rate of fatal overdoses would be significantly higher, since benzos make opioids far more lethal, thus wiping out the customer base.