that both parties routinely make expensive policy changes 'temporary' for budgetary reasons - the total math only adds up if the change is calculated only for a limited number of years. The intention is from the start to make said change permanent, it's just it will be future Congress' problem to figure out how to pay for it. Though there may also be a political factor to it - if the change in question is one that generates particular enthusiasm among their base, it may be politically convenient to have not only the initial vote that passes it into law the first time, but also a later second vote on renewing it shortly before or after some future election. And given the strong loss aversion people in general have, it's usually a more difficult vote to take away something they already have, than to refuse giving them something new.

Of course, if one believes as you do that Obamacare is a failure, many of those points don't apply - but while opinions may vary on whether its benefits outweigh the costs overall, certainly those benefits are very important to many millions of Americans who can only afford healthcare insurance, or can only afford to have it cover specific conditions, thanks to Obamacare and the Obamacare subsidies (which as I'm sure you remember were intended to be there from the start, but at the time the Dems didn't manage to get that far).

That is true to some extent, but only one party has become the spineless bitch of a demented would-be dictator who has never even bothered to pretend giving a fuck about the US or the interests of its people, or about doing the job he was elected to do. Some of the fights between Dems and Reps in Congress, including this one, would probably still have come up if there were a Republican president worth the name right now, but still there is a large group of people who might ordinarily be open to the point of view of Republican Congressional leadership but now can only think that Johnson's or Thune's word is worth literally nothing whatsoever. Nor is Trump's, though in his case not for lack of power but rather because on any given day his position or the 'facts' in his mind will be randomly made up without reference to any actual facts or prior negotiations.

And ultimately he is also the reason that the shutdown is taking so long - the Republicans in Congress know very well that a deal will need to be negotiated where each side will have to give and take a bit, but since they are enslaved to a president who doesn't have the attention span, interest or perhaps even mental capacity of being involved in negotiating said deal but will be happy to randomly attack various parts of it and throw whoever negotiated it under the bus, they don't dare to actually negotiate. While Democrats know that their supporters understand all this and hope that they'll be willing to bear the consequences of the shutdown long enough until a large enough number of swing voters and non-voters finally wakes up and puts more pressure on the Republicans. If that ever happens.