The tunnels may seem kind of ridiculous given that we haven't built them where they're more appropriate - downtown Mpls, under the Snelling intersection, etc. But transit reliability is generally better in a tunnel, and this is already funded and rolling forward so I can't imagine any logic in revisiting the decision at this point. It points more to the absurdity that those other tunnels *weren't* built that to the absurdity that this one *is* being built, in my opinion.
If the region were given a hypothetical $6 billion in one shot to have transit experts design the four core lines of a transit system, it's plausible that the Blue Line, Green Line, Green Line Extension, and Blue Line Extension would be the four core routes (though there's a case for the Riverview line in that scenario). It's not likely that the tunneling money would be invested primarily in the Green Line Extension; such investment would probably be made at the Airport, DT Minneapolis, the UMN campus, and maybe the Snelling intersection, and some bridging decisions would be made in a variety of places where they weren't.
But of course that's not the way it worked. The Hiawatha Line was designed as basically a cheap experiment to see if the region was really interested in light rail, and the Central Corridor was designed as the core backbone of a transit system when we had no dedicated regional money and no obvious path to get it, and the Bush administration was reluctant to make speculative transit decisions. Southwest, designed with a much friendlier federal administration in power, ample regional transit funds, and a core transit system already in place that was drastically exceeding ridership projections, made the possibility of solving arguments with tunnels politically desirable. It's not a bad outcome, it just makes the infrastructural investments a bit lopsided compared to the justifications for those investments.