If we're controlling the cells' genomes (which we are), we can add any sort of killswitch (see another comment https://news.ycombinator.com/item?id=45220068 ) that we would like, and this would function better than relying on host immune surveillance. The opposite could be done, making insulin release dependent on the presence of a harmless drug, e.g., insulin release can only happen if a designer steroid molecule is present in the blood.
There are already cell therapies that envisage permanent implantation of modified cells, so I am not sure why a long delay for 'any type of usable treatment' would occur. The structure of this need not be analogous to a stem cell transplant; you could imagine injecting new cells intramuscularly every few months.
The costs to develop this are incurred during development (unlike the autologous therapies that require extensive, expert-level analysis for each new patient). I'm not sure that we can compare the current levels of investment in autologous gene editing to this product.