As someone who has worked in IT I know very well that deadlines are suggestions at best. (In some sense the concept of deadlines for large features goes against the spirit of agile methodology anyway.) It is very hard to know how much work a large job really is and there are _always_ unforseen problems. In addition, your ability to estimate and predict future work declines with project complexity. Having anywhere near a constant cadence is a bloody miracle, especially for a project that already has so much stuff already implemented. And re-implementing something from scratch in a new technology? Oh boy. Im not 100% sure what benifits this new stack offers, but I am not at all surprised or disappointed about this delay. Take it easy, I have nothing but respect for your craft.