You just answered your own question right there.
We've already seen plenty of examples of new restraints that do away with OTSR's for looping coasters - Premiere's lapbar system, Vekoma's "vest" that they've put on an SLC and a couple of loopers, the X-Car restraint, etc.
The biggest issue here is retro-fitting an old coaster would cost the park hundreds of thousands of dollars to do this. So unless they are wanting to re-market an older ride, like Morey's Piers did with Great Nor'Easter, I don't think we'll see this happen.
I would hope that any future Premiere, Vekoma or Maurer Shoune installations come standard with their updated versions of lapbars for looping coasters, and it would be nice to see some other manufacturers, like Gerstaluer or B&M for example, follow suit.
But I also know that developing a new restraint system, when you already have one that works just fine for the parks, is VERY expensive to put into production. So unless a park is willing to fund that development when ordering a new ride, I doubt that's going to happen either.
So to answer your question - Why do we have them? Because it's expensive to do anything else.
You are correct.