There aren't any threats, support for win9x, winME was meant to have ended over a year ago.
What is giving it life blood is 4.8 is still supported for the business range of avast products (avast 4 server, ADNM, etc.) don't currently support the new avast AV versions. When the new updates to these Business products are widely used the new versions of avast AV will be supported.
So as that progresses apace there will be less and less 4.8 share and it will have life support turned off.
Those basically are my best guess as to the reasons why win98 is still supported, not by design, but a fortunate circumstance. But this isn't something that you can count on lasting for a long time, enjoy it whilst it lasts and do some forward planning (finding a new AV, upgrade your OS or system, etc.).