It is one of the common growth models, but there are risks and other options. e.g., If Insomniac were to go cross-platforms, their existing skillsets and edge will need to be re-evaluated (Their cost and performance may take a hit).
If they license it out for someone else to port at no risk/cost to themselves, then it may be fine. OTOH, remaining PS3 exclusive allows them to manage their cost and honed their skills further -- provided the PS3 growth is healthy.
With their understanding of PS3, they can also explore other businesses within that space (e.g., Explore other genra, Blu-ray related, or Home related, or other stuff). Not everyone needs to be big. Some huge companies make less profit than smaller ones.