Your battery will fail, it is just a question of when. When it does fail, you may get some advance warning (slow to start), but if you don't you will be stranded. The reason to replace it is to control the time when you replace it, ie in your garage and not in a parking lot somewhere on a dark night. Only you can decide what the inconvenience is worth to you.

If it costs $120 and lasted six years, that is $20 per year. If you get another year out of it, you can reduce that $20 to $17 per year. But if you have to pay for a tow or a jump, or even a taxi ride home, you won't be better off.

After six years, I would replace it at a time and place of my choosing. But I wouldn't spend $300 to have it done at a dealer.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue