For the past 15 years (except for my wife's latest car), I have bought only 100,000+ mileage Japanese vehicles. My latest I went backwards and sold my 96 Altima 160k miles for $50 to buy a 94 civic with 180k miles for $69 (only paid title and registration

) It was better maintained than my Altima was. My wife has a brand new Ford which is a new experience in owning something new and something American.
There are 2 issues here.
First issue is obviously the reliability problem with an older car. You have 2 older cars. Something is more likely to happen to an older car to get you stranded somewhere or just stranded at home unless you have a backup. In your case, both of yours are older and the potential of both being disabled is higher than if one of them was new.
The 2nd issue is the one that hurts me. I wanted a new "reliable" car for my wife and kids to be in. I don't want her by herself with the kids in a 150-200k mile car. But with having a car payment on a new car, the funds aren't easily there to keep the older car running in top condition. That was the issue I had with the Altima. I'm not a mechanic and hate working on the cars, but I couldn't afford the car payment
and pay for any work needed on the Altima. $100 for me to weld a new front radiator support on opposed to $1000 for someone else to do or $0 for me to diagnose the engine light and try to get at the sensors crawling around in the gravel driveway opposed to $500 for the shop to do it. Both of those put my car down for a good while, but I did have the backup of a "new" car to get me to work.
All that said, it also needs mentioned about what kind of car you have / are looking to buy. As I said, for the past 15 years, I've only bought Japanese vehicles until my Ford. My Ford failed state inspection with the brake rotors being completely shot at 35,000 miles, all 4 corners. At the same time, the Altima was also needing front brake rotors. The difference was, the Ford rotors were completely shot at 35,000 miles and needed replaced while the Altima rotors that needed replaced were also the original rotors with 160,000 miles on them. Big quality difference.
So far, my "new" Ford with 35,000 miles on it has cost me more money in maintenance than any other car I have ever owned the last 15 years. Here's the list of my last 15 years of vehicles and maintenance besides tires and brake PADS...
89 Toyota pickup - 139,000 miles (new shocks)
94 Toyota 4Runner - 100,000 miles (nothing)
88 4Runner - 189,000 miles (new clutch, $100 installed myself and new shocks)
99 Tacoma - actually did have less than 100k on this one at 92,000 miles (nothing, but it did have the rusty frame problem that Toyota fixed for more than I paid for the vehicle)
96 Altima - 160,000 miles (needed shocks, but I never replace them)
The Altima and my "new" Ford are the only vehicles I've had to replace rotors on. I was appalled at having to replace the Ford rotors at 35,000 miles. The 88 4Runner I did have them cut down once, but the Ford was even beyond being cut down.
Moral of the story is, just because a car is new, doesn't mean that it will be cheaper with regards to maintenance. You also have to consider what the new car is going to be. Replacing "normal wear items" at an increased rate of 5X is going to be considerably more expensive.