Bulk SMS spam would most likely come from someone with direct signalling access and not from individual SIM cards which would be trivial to detect and block by the operator.
It is trivial, even for someone not that technically oriented to send a mass SMS from Android, with the appropriate app. Since it's easier to sideload on Android, it would be even easier for a malicious spammer to pay people to install sketchy APK's that spam from the user's phone relentlessly.
This would be simple for the user to execute but it would very quickly be spotted by the operator as it's all from a single originating MSISDN.
Spreading the load over many users like your latter example would be a lot harder to spot, as would spamming through multiple SMS providers as you're diluting it (but it might also get picked up by the provider e.g. Twilio, MessageBird etc).
My point was that most spam originates from people with SS7 access and not SIM cards. It can also come through low cost SMS providers but is short lived as it's blocked the moment it's discovered or there's a complaint.
How does one even obtain SS7 access w/o a AUP forbidding this kind of abuse? Or are the Telco owners making more money not caring @ that connection level?
What's more, all you need is a broadband dongle and you can send SMS with a simple script straight from your PC - but as others already said, it's hardly a real source of spam.
I think there's an increasing amount of SMS spam being sent by random compromised consumer devices, which is probably what drove T-Mobile to take this sort of desperate measure. It would seem like notifying the customer is even more warranted in this case, though.
