Sure, filters extend the life of oil, but they do that by using a porous filter media. The larger particulates get stuck in the filter media, and the cleaner oil exits the filter...until all the microscopic holes get filled with particulate matter. Once the filter media is clogged, one of two things happens:

• on cheap and horrible filters, oil goes into the filter...and never comes out. The engine runs dry, heat increases, and you spin a crankshaft bearing or blow a head gasket from the heat generated by increased friction. Engine dead.
• on a good filter, the increased oil pressure from the clogged media forces open a bypass valve in the filter so oil (dirty unfiltered oil) goes straight through back into the engine. The concept being "dirty oil is better than no oil at all". However this means that the filter is now completely bypassed and the engine oil will continue to accumulate dirt and matter eventually clogging fine oil galleries and causing localized engine starvation of oil, spun bearings, increased heat, etc. Eventually, dead engine.

Oil filters aren't an infinite life extension for oil. They help certainly, but this help is built into the maintenance schedule for changing your oil. Unless specifically noted by your manufacturer don't assume you can change just the filter and your old oil is fine.