At 15 years old, you definitely got your money's worth out of it. Most likely you've lost a cell or the buss bar linking it if it died suddenly. 12V batteries are 6 2V cells in series, so if you lose your buss bar, or one of the cells fails (usually the plates detach from the bar) you'll lose the whole battery.

The battery maintainers DO help with vehicles that sit parked for long periods of time. If the car is driven regularly, the charging/discharging cycle of using the battery helps keep it desulfated. The SL got ~10 years out of it's last battery while kept on a maintainer, while a Ford Explorer that sat for the same period went through 4 batteries since it was NOT kept on a maintainer.

What kills a battery that sits is low charge level and temperature. The acid undergoes a chemical change and actually deposits sulphur compounds on the lead plates (insulating the plates and diluting the acid concentration). The maintainers cycle the battery and keep it in a range that it won't form the sulphur precipitates. Some of them (like the one I use) can even help reverse a light precipitate coating if the rest of the battery is healthy enough. It won't save a wrecked battery though.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]