My offhand guess about extending the deadline for enrollment in the exchanges is that there is some point at which the insurance companies will balk. It also seems to me that it raises a takings issue (if you take the insurance model at all seriously).
What you may not be taking into consideration are the risk corridors put into place, acting like a 3 year safety net for insurance companies. In this way, any fiscal strains can be compensated for by government bail-outs. For at least a few years it’s a win-win arrangement for the government and it’s insurance company cohorts, to have time for PPACA to become firmly seated into place.