Marvin's rocket constantly had problems. I don't recall ever seeing it operate, and it sat for a long time 'under maintenance' before it was quietly removed.
Taz's cars - i'm not sure. It was taken out at around the same time as JDS was installed - perhaps given the similar ride experience, and the labour cost, it was easier to close it down. The Taz cars were also getting on in years, so probably coming to end of life, and cheaper to remove when JDS provides a similar experience.
Taz's former site is now a parents room \ area, and its a great facility for parents with small children - and far better than their previous space. Whilst it may not appeal to everyone, its perfect for that age group, and should remain where it is - the facility won an award, and is a much better pit-stop for parents with infants than any park i've ever been to (although i've never seen a Disney parents room).