I too have v13.x and have used v10.5 for years - still do. I have not had any issues with external change impacting the v10.5 models, not even Office version changes. I still have customers using v10.5 models for monthly reporting. Now that I have said that, I can say for sure that if the model is mission critical, I use the latest version. It would seem that converting a model from one version to another should be easy, but I have not had that experience across the board. Some are truly a slam dunk, but others cause a lot of pain.
To echo Joe, for most applications v10 still runs fine. The risk is low as long as you control the operating environment - so if you can keep an XP machine running as a VM, you're likely to be safe. The problem is that most corporate IT environments might not look kindly on such a solution and would prefer that the application ran on a machine with supported OS, Office, browser and anti-virus, and some might even insist on a supported application, which forces you to v12 or above, with the consequent loss of desktop automation and the conversion to the binary dmod format.
When converting models, or even when opening xmod and saving as dmod, you might hit some snags around definitions that depend on each other, such as complicated calculated fields, and compound filters, but these can usually be solved by editing and saving the object definitions in a logical enough order until Monarch understands what's going on.