If they want to protest, fine. I support their right, however when they start blocking roads and harassing people etc, then I draw the line. Do we need TMT with Hubble and Keck at our disposal? Not qualified to speculate but seems to me maybe we don’t need the TMT. 
I'll try to answer your questions, I've been out of the loop for a few years but all the rationals still exist.
" Do we need TMT with Hubble and Keck at our disposal"
Hubble is nearly dead and is barely working, I think if one more Gyro fails it will be useless.
The replacement, The James Web space telescope is way over cost and they are having some
very real technical problems, I'm not sure they will every get over them.
Keck I and II are awesome facilities. Fairly up to date.
They had a mirror cracking problem I'm not sure they have fixed yet.
There are also limits as to what they can do to update the telescope itself
to deal with Keck's physical limitations eg wind shake. The summit gets very windy.
In Astronomy the more Photons you can gather
repeatedly on the same detector Pixel 13 micrometers( or less) across the better.
Keck is ten meters across. TMT is thirty meters across. that is a significant increase in
area to gather photons. TMT is also designed to compensate for problems large
telescopes like Keck, et al have.
The Astronomical community figured the billion dollar Plus cost of the TMT is worth the effort.
India, China, Japan, Canada, et al seem to agree as they are heavily invested in the TMT.
Do we need the TMT? In astronomy all you are doing is studying history.
Studying galaxies that may not exist anymore as their light took 13 billion years to get here.
What good does that do?
Why go to the moon? Why go to Mars?
The answer is because somebody is willing to pay for it,
because they want it, not because they need it.
Personally I thought the TMT money would be better spent
eradicating cancer, but that isn't as sexy as finding binary
Black holes in a far off Galaxy.
Hope that helps some.