The Hummer was designed in the 70s? Issued in the 80s. Before soldiers wore so much gear (mainly body armor) during operations. The seats in a HMMWV are tiny. There is very little room for a modern soldier with full kit. And the poor truck commander (TC) now has masses of communications equipment in his lap and face as well. If a modern armored HMMWV rolled onto the passenger side, you might as well say your prayers for whoever is in the front passenger (TC) seat.
Then we get to the crew protection problem. HMMWVs are aluminum. They provide basically zero protection. And in the urban warfare of middle east contingency operations, that was a no go. So we start strapping thicker and thicker armor onto them. Now the drivetrain is being overtaxed. Okay upgrade the drivetrain. But, the HMMWV, no matter how much armor you put on it, is still going to have a flat bottom and that is a fatal flaw against IEDs and mines. Oh and all the armor only makes it even harder to get in and out of for the Soldiers.
So yeah, the HMMWV is obselete.