Last year they sold over 250,000 MiVoice 250 systems. The product makes up a huge portion of their sales, and it isn't going anywhere
6900s are now supported on the product via an application server, meaning that Mitel will eventually streamline to a single series of phones for the 250, Connect, Business and all Cloud Products (Connect Cloud, MiCloud Business, and Flex).
Does Cloud cannibalize some of the 250 sales, sure (as it does Connnect and MiVoice Business). Can they afford to turn their back on 250,000 system sales, that also generate on going revenue, the answer is no.
It is a great product, and new development is still happening every day for it.