Does your wife have to drive it? If not, why does she even care? If the car you drive makes you happy and isn't costing a fortune, keep driving it.
Some people think it's new car time as soon as a windsheild wiper blade needs to be replaced.
New cars break too, sometimes catastrophically. Even if covered under warranty, there's still the time inconvenience. I have seen 2013 model cars get new engines and transmissions. My 13 year old truck has never needed that, and has never put me on the side of the road save for one tire blowout (caused by metal in the road, not the tire's fault), which could happen on a brand spanking new car.
Having to repair a 17-18 year old car about once a year sounds pretty good to me. That's a solid track record, assuming you aren't talking an engine or trans every year. A new car only buys you about 3 years of not having to do anything but oil changes assuming the car doesn't have a warranty issue.
A simple way to calculate is: what will a new car cost per month vs. what will the old car cost per month (including time)? If the old car still costs a lot less, the new car is a want, not a need.