At the very least, you need a replacement that's technically better, works well cross-platform, has a layman-acceptable UI, and supports 99.999%+ of all the use

At the very least, you need a replacement that's technically better, works well cross-platform, has a layman-acceptable UI, and supports 99.999%+ of all the use cases PDFs currently supports. It also has to convert old PDFs into the new format. Then, you have to worry about market share and acceptance.