I'm not even sure it's an "alternative" hypothesis - it's sort of the obvious conclusion, in terms of means, motive and opportunity, that the US sabotaged the pipeline. I bet a poll of US citizens would show a clear majority believe the US to be the culprit, but a minority have a problem with it. And so you get this situation where everyone kind of accepts the truth but different people have varying levels of enthusiasm for publicly arguing about it.
Even extending to the international community (which is generally a reflection of American politics, much to the chagrin of Europeans who are unwilling to admit it), there is mixed levels of enthusiasm for getting to the truth. The countries are all anti-Russia, and so the pipeline sabotage is generally seen as a "good thing" except by the countries with direct profit motive for it. However, those countries aren't about to publicly accuse US intelligence of carrying out the operation, because their relationship with US intelligence is too important to lose, especially given all the weapons and intelligence they provide.