Q.indoor lemon tree scale
When is a plant too far gone? My lemon tree started out strong about 4 yrs ago. I was able to keep it outside in summer the first year. Second year it returned indoors with a bad case of scale. It appeared to recover but I decided against it being outdoors.
Scale has returned with a vengeance. All leaves gone, no new growth in sight for weeks. I’ve been spraying and wiping with soapy water. I recently changed out the soil.
None of the above seems to be helping anymore. It could be dying. What can I do? Would changing soil again and washing the (abundant) roots thoroughly be helpful at this point? What type soil is ideal?
Thank you

Typically, neem oil will take care of this issue with a few application. As for what's happening to it... I have many container citrus myself. It took me a long time to figure out how to keep them healthy. The answer is as simple as some dolomitic lime, and iron sulfate. This will prevent most issues well before they start. Container citrus tend to change their soil environment very quickly, so applying these once every few months will mitigate the changes the citrus will make to its soil.
This article will help with growing container citrus: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm
This will take you to articles on many ways to control insects, naturally: https://www.gardeningknowhow.com/special/organic/