• Security
  It is unmaintained upstream, for years in case of Qt.
• Functionality
  Well, I think @rinigus and others could write long lists of functions in newer Qt versions, which would make programming for it much easier, plus the loss of interoperability of software not specifically for SailfishOS, because all other Linux distributions are on a way newer Qt.

Both aspects were the main incentives for Jolla to update most of SailfishOS' basic tools and utilities (GCC & Co., GNU-utilities / Busybox etc. etc.) over the past two years.
This is why it is obvious that they have to resolve this, and I wish this would happen rather sooner than later (even better: long ago!), because the severeness of the impacts of this issue are permanently increasing (until on a Qt version, which is maintained upstream).