I'm not Civil, but I would think there is almost certainly something in one of the USDOT CFR titles that would require them to shut down the Eisenhower Tunnel were they to work on expanding it for safety of motorists. The fact that I70 carries so much commercial traffic means it's almost impossible to shut down the tunnel. Even if they aren't required by regulation to close it, I'm sure they would. I know there were major problems when they were doing the original bores and they ran into problems with slip planes existing in the actual Loveland pass mountains.
I couldn't even the budget to rebore those tunnels to support another lane or two of traffic each. It would have to close to a billion dollars. Plus I have no doubt they'd go overbudget and overschedule on such a project, further delay commercial traffic it's most effecient route.