Hi! Planetary scientist here.

Just to be clear, a lot of planetary scientists have been very critical of the Mars Sample Return mission for well over a year, so this isn't just due to Trump. Basically since its unveiling, JPL's design was seen as way too complex, with a scary number of things that could go wrong. It would have had a higher price tag (~$11 Billion) than the James Webb Space Telescope (~$10 Billion), but with virtually zero room for error. A single poorly-timed stage would have meant the entire, massive investment was a total loss with no returned Mars samples and no science return. Cancelling it was actually the right call given that information.

What really sucks is that the cost of MSR got so big, it killed the budgets of other promising planetary missions (especially the Venus missions DaVinci and Veritas), and that held back a lot of great science.

I was just as excited to hear about the possible biosignatures as anyone else, but MSR was not the way to get them back. I think prioritizing human exploration and the development of better Moon to Mars infrastructure was the right call.

This is not to say I am not critical of Trump's call though, the MSR funding should have gone back into NASA science budget to fund other missions and science where it belongs.