It is simply a case of Stelvio versus Hafling/Avelengo as the mid-stage climb there, Collalbo and the correct side of Pinei are not mutually exclusive with Stelvio whatsoever, so you don't get a better stage by not 'wasting' Stelvio. Also, the fact that the tunnel was possible in 1972 doesn't automatically mean it still is over half a century later - a) there is much more traffic in and out of Livigno nowadays, so the effect of closing the tunnel would be more severe, b) there is no sporting benefit to going around Stelvio (unlike in 1972 when it was the only way of using Stelvio from Prato), c) the Swiss authorities might also simply not be open to it anymore for all we know, and d) it would still be a bigger logistical hassle than just going over Stelvio and thereby not having to close a tunnel that already has capacity issues whilst also crossing a Schengen border twice. For all those reasons, it isn't really a credible alternative, IMO.