Plants that are under stress for other reasons, such as improper light and water, are much more susceptible to pest infestations. Replacing soil and rinsing roots is one of the more stressful things that can be done to a plant. As Bill noted, it is not an effective treatment for mealybugs anyway. It is likely that those extreme treatments are what has caused your plants to die, not the mealybugs.
Diluted alcohol and liquid dish soap is an effective treatment but only if direct contact with every single mealybug is made. That is not easy to do unless you spray so thoroughly that every leaf and stem and nook and cranny is literally dripping wet. The solution has to wash over all surfaces and into all spaces. It is a very messy task best done outside for that reason. If you are really thorough, one treatment is usually effective. A followup spot treatment might be necessary if you miss a few and catch them early in their return.
That said, you may eradicate the pests but lose the plant to whatever the underlying condition is that is stressing the plants. Healthy plants with proper light and proper watering are quite resistant to most plant pests. That should be your first line of defense and treatment.
