They still are around but don't do tours. They own all of their music and release new CDs of concerts along with anniversaries.
They retired from the music scene officially in the 1980s but will make special appearances. They did do enough songs to last a lifetime.
There was a teen group called A*Teens which covered the work of ABBA. It was successful for the first album but the novelty wore off quickly.
I hear there is possibly going to be a Mamma Mia 2, but it is up to Meryl Streep and her schedule. (There are enough songs not in the first one to go into the second movie.)