Risa

It's just bad writing. They could literally all come back to earth, get the same injection Tom and Janeway had and in a couple of days they would be back to Earth. That is without considering that warp 10 was a side project of a lone helmmen stranted on the other side of the galaxy, just think what the best federation scientists would do with the appropriate reaources, in a couple of years they would eliminate any adverse effect.

You can't just drop one of the most relevant scientific breakthrough of the century as a story for a single episode and then completely forget about it.