I’ve talked to so many people who have lost weight (intentionally) and kept their now-too-big pants “just in case.”
If keeping the weight off is a priority, get rid of the pants (or get them tailored to fit). By keeping the pants, you’re giving yourself permission to gain the weight back.
(The exception would be pregnancy-related clothes.)
Likewise, if you want to stop gaining weight, stop buying bigger pants.
In both cases, when what you have starts to get snug, that’s a heads up that you need to get your habits back in check. If your habits have legitimately been in check, then it’s time to see a doctor.
No, they didn’t shrink in the wash.
Elastic-waisted pants will not help you with this. Which is not to say that there’s no place for them—they’re comfy and wonderful!—but pants that hold you accountable need to stay in the regular rotation.
A free tool, right there in your closet (or dresser). Use it!