I think your point that text messages are equivalent to phone calls is very revealing. If so then a text message should be part of the phone plan already. Perhaps what needs to be done is to have text messages each count as a 15 second call against your minutes.
That's not a bad concept.
We'd also have to account for store and forward centers, though. And the whole half-SMS, half-data mess that MMS is.
A problem is that, starting a year or two ago, people now spend more minutes texting than on voice calls. Sometimes during the day carriers are handling millions of texts a minute.
So the carrier infrastructure had to change. Now carriers need more SMS store-and-forward centers, which means real estate to put expensive server farms, with support people and backup power and everything else. It costs.
But here's the worst problem with so much texting, and it's also a national security risk: network paging overload. While SMS centers can be built to accomodate increases in texting, cell tower control channel bandwidth cannot.
As noted previously, texts are like mini phone calls. They require a phone to be paged, the phone to acquire a channel and authenticate itself and the network, and then the text to be sent down to the device...and finally for the device to send back a receipt acknowledgement.
A side complication is the way networks page phones. They don't just use the last cell you connected to. They send the page to a group of cells in that area, in case you've moved around. This increases the control channel usage in more than just one cell.
The upshot is that short message texts are overwhelming networks that also have people trying to make voice calls. It'd almost be better if SMS was removed in favor of data-based IMs, but there's too many dumbphones to do that.
Mobile networks are complicated creatures, with lots of little bandwidth interactions that add up to strange restrictions at times.