Three years ago I bought a 'rare' philodendron from a pop-up market in Tampa, and it's still spreading mites to my other plants. | Botany Lovers - Vivisector
Why do we keep things that only cause problems? I see this with stuff in my house, not just plants. We hold onto a broken appliance because it was expensive, or a shirt that doesn't fit because it was a gift. That plant is a three year long problem you're paying for every day with your other plants' health. Sometimes cutting the loss is the smart move, not a failure.
Honestly, throwing it out feels like giving up. Some plants are worth the fight. I've nursed a few back from the brink with consistent sprays and isolation, and the payoff feels huge.
I get what you mean about it feeling like giving up. I had a fern that looked totally dead, just a sad brown stump. I kept watering it on a strict schedule and talking to it like a weirdo, and it finally pushed out a new frond after months. That little green shoot felt like winning a war.