@ReformedEndralon
Sorry mate, but your analysis misses the mark by a mile.
SD did not start to grow in some vacuum left by the Moderates as the latter moved to the centre*. The old Moderate party was all about cutting taxes for the upper and upper-middle classes (their precursor the Right Party was a true traditional conservative party that cared about family, crown and fatherland, but that was a long time ago). SDs entire raison d'être on the other hand is stopping mass immigration from the third world, and they have pulled voters from the entire political spectrum united over this one issue. Why? Because immigration is a black hole (no pun intended) gobbling up all our resources. This means worse infrastructure and public services for everyone while the tax burden gets ever more suffocating, so all classes are hurt, but especially the working class and the middle class.
You are also wrong about the FPÖ. The FPÖ did lose a lot of votes after being in a government coalition in 1999-2002, but it's now back to the same level of popularity again. Furthermore they were in government aldready back in 1983 and it was after that that they really started to become big in the first place. There is no statistical trend here. It's fairly normal for all parties in government to lose some popularity compared to before the election. Comparing FPÖ to UKIP or SD is nonsense anyway since they are not some young upstart party - they've been around since the 50s.
For an example of "populism" in power that shows no signs of "cracking under the pressure" one need not look further than Fidesz in Hungary. Also consider Denmark where the Danish People's Party has been so successful that all the mainstream parties have adopted their politics and are now arguably more "xenophobic" than SD.
You are right in one thing. SD will be under a lot of pressure once in government, because as I said, immigration is the one single issue holding the party together. What I, and most SD-supporters, hope is that SD will get into power (directly or indirectly) and do what the DPP did in Denmark. I couldn't care less about the future of the party itself. A party is only a means to an end, and in SD's case that is stopping mass immigration. Once that is accomplished SD is superfluous unless it reinvents itself - they have sort-of started to do this by embracing social conservatism in their latest manifesto.
Actually, SD has potential to do what the DPP did but even more thoroughly. Swedish people are very hive-minded which is why political correctness has been so suffocating here, but this also means that once the pendulum finally swings in the other direction, "everyone" will turn 180 degrees in perfect uniformity, and the fact that for the last decades we were doing the opposite thing will disappear down the collective memory hole. Debate is the exception in Sweden and consensus is the default, we are merely going through a transitional phase from one consensus to another. From fanatic multiculturalism and ethnomasochism to fanatic nationalism.
This is a kind-of creepy aspect of the Swedish Volksgeist.
*There is actually another, as of yet very small party, that better fits that description:
Medborgerlig samling.