"Deprecation" is a difficult task, and "abandonment" even more so, when you do not have a legal contract with the supplier of the software upon which your system depends. In the short run, you can indeed "remain right where you are," as long as the system is running, but obviously you will soon have to make a very painful move. It is obvious that Mongo, like so many other companies, now wishes to force your company to subscribe to their "cloud" offerings, and that they intend to cut off all other options to compel you to go there.
In the shortest run, you can continue to use the (now, abandoned) binaries that you now use. In the longer run, though, you will now be confronted with ... what you say is "not an option for now." A major and unwanted application restructuring, first to isolate and abstract the parts of the present system that are today "Mongo dependent," then to replace that technology with a more defensible replacement.
No. Mongo's present Wall Street investors did not intend for your path-forward to be easy.