In fact, the reverse is true. It is far easier to recover your Family Tree Maker files assuming you have a backup procedure in place than it ever would be to replace your Ancestry tree if Ancestry had a major service failure.
I use an online service to back up all my PC data. This is done automatically - there are a number of them that are good.
If you find you have to give up your subscription to Ancestry you will lose the images from their databases that are linked into your tree. You will still have your tree, and still have access, but you will no longer be able to look at all the images, just the free ones.
This is another reason for downloading the images to local storage, either within FTM or outside it. Come the day you have to drop your subscription you will still have the images stored locally.
Additionally, FTM has more features than Ancestry and probably always will (at least for the foreseeable future).
So, start thinking of FTM as your primary means of storing your data. Set up a good backup plan for your data (all of it). When you are finished with FTM for the day, Compact the file (found under tools) and create a full backup making sure that you include the sync controls in the backup file. Compacting your file before syncing is always the best way to go. NEVER let Sync occur automatically, set it to manual where you have control of the process and always, always review the log of changes before letting it continue.
With these little things in place doing a restore from an old version is relatively trouble free.