Exactly. And the whole point of a cellular network architecture is that it's resistant to DoS attacks (what the rubes call "unexpectedly heavy usage"). Sure, you can take a cell out with a hundred fake phones, and all the users in that cell will hop to the next one. Or at worst walk a block over to find another. The attack doesn't scale, at all.
And even if you wanted to deploy custom hardware to do it, it would be far easier to just use a high power jammer on the band anyway than mucking around with all those SIMs.
These are for making actual use of the telecom facilities at scale, with the anonymity you get from burner SIMs. It's fraud, not terrorism.
Some parts of it are (DoS resistant.) And some carriers are more resistant than others. Verizon's CDMA from the 90s / early 2000s was NOTORIOUS for falling over when too many people texted at the same time. But yeah, it's been a while since things were that bad.
And even if you wanted to deploy custom hardware to do it, it would be far easier to just use a high power jammer on the band anyway than mucking around with all those SIMs.
These are for making actual use of the telecom facilities at scale, with the anonymity you get from burner SIMs. It's fraud, not terrorism.