The costs of trying to turn a novel therapeutic approach into a real therapy are extremely high - hundreds of millions of dollars. I'm unaware of any approved therapy that utilizes protein transduction of cells - I also suspect existing protein transduction methods aren't very efficient. There is a tiny pile of evidence that this method "works" in vitro in cultured cell models of infection, I can imagine a hundred ways it will fail in bodies.
There's a reason some ideas are left unexplored by industry.
But your argument essentially boils down to "We haven't yet discovered an effective delivery method, therefore this technique will never work".
Isn't that one of the basic problems facing all clinical genetic modification research? Is it unreasonable to assume that this problem could be solved by some future breakthrough, or does it somehow violate the laws of physics? If so, should we then discard all basic science research in this field because there is no clear route to market?
If there's an ELI5 (or, ELI-college-101) I'd be interested to read it.
There are actually two separate systems: the T cells and B cells. I recommend the very readable Lauren Sompayrac's "How the Immune System Works". Or google/wiki "clonal selection" and "VDJ recombination".
There's fairly recent technology to sequence these antibodies en masse, which gives you a whole load (~10^6) of these antibody DNA sequences. It's a fascinating and frustrating exercise to try and reconstruct the mutation history and families of related cells from this data.
I think in the sense you are asking, though, is that any long-lived plasma cell or memory B cell that is active will probably not change to the extent that they would attack self. I don't know off the top of my head if there are examples of this, but I can't think of any.
Sure, these people can be kind of annoying, but I think we lose more than we gain by discouraging cross-pollination between fields of science.
It is a tough call to make between what's happened and what's possible.
Experimental new treatment not yet feasible? Let's invent a whole new synthetic organ that might not even solve the problem!