I have long felt it is a missed opportunity for an organisation going through a post-merger integration journey to focus purely on integrating two current business models. After all, what do you end up with 2-3 years down the line other than two antiquated business models beautifully (at best) integrated, when competitors have evolved their business models in line with evolving market and customer needs over the same time period? 

Far better to take the time up front to define a target business model for the combined organisation, regardless of legacy, and take this unique opportunity to integrate both organisations to the "greenfield" business model of the future. If you don't, then you will only have to go through a second transformation post-integration, and who really wants that level of disruption and wasted effort to go through the rigmarole twice over?