Zorin OS is financially sustainable and well supported.
The ZorinGroup has openly stated that they fully intend to continue developing Zorin OS long into the future. While I could state it would take a major catastrophe to stop development of Zorin OS; even the invasion of the land of their forebears was not enough. In fact, Zorin OS was quite a boon to the Zorins for that effort.
While some package maintainers may lose interest and abandon projects sometimes... The ZorinGroup has stated that they have set goals and will not suddenly abandon the project. They do Zorin OS development full-time and have no other pressures or distractions that directly inhibit Zorin OS development.
It is not a side-project... it is their livelihood.
You can expect Zorin OS to be available long into the future. What is actually more interesting is in us all learning the Direction of Zorin OS.
The question is less, will Zorin OS be there, as much as, what will Zorin OS be like in the future?