The same engine used in BX's is used in industrial applications such as generators, portable lighting units, etc., where it's not uncommon for those engines to rack up 10,000 hours and more. The key is sensible operation and good maintenance. The other critical - and costly to replace - component is the HST drivetrain. The HST's are also quite reliable, but the same factors apply to keeping one healthy for the long term.
The rest of the tractor is just like any other tractor... hoses, bearings, seals, mechanical parts, etc., all wear out with use. Repair/replace as needed to keep the unit running.