The G'mara has 2 l'shonot re: Hasibbah for the 4 Kossot
One lashon is hasibbah davka the first 2 kossos
The other lashon is davka for the last 2 kossot
Now since this is a safeik D'rabban, why bother with hasibbah at all - mimah nafshach? After all each Kos is a Safeik D'rabbanan?
One answer given [see Schottenstein] is that since this would de facto engineer NEVER having hasibbah at all for ANY kossot, therefore we still need to be machmir and do it for all 4 misafeik so as not to be m'vateil it completely.
Now let's go to a REALLY cold village in Russia. There at the end of Tishrei, every yeshivas sukkah could be construed as "mitzta'eir" by definition. Do we exempt everyone for all 7 days?
The lamdut from the above case of Kossot says NO!. Mitzta'eir is a p'tur only when you can still SOMETIMES be m'kayyeim sukkah [EG Rainy Days vs. Sunny Days] but if it would obviate everyone's Hiyyuv for every day, then we "punt" and require sitting daily - unless/until other factors intervene.
Notice, that this Hiyyuv would NOT be extended to Sh'mini Atzeret.. And in those climates once one was m'kayyeim the ikkar haddin during the first 7 days, mitztaier could easily kick in for the s'feika d'yoma.
[I Believe I originally got this s'vara from the Aruch Hashulchan.]
This leads to a Kulla and a Humra
Humra: Namely if the climate was the P'tur for those in Eastern Europe to not sit during Sh"A, then their descendants in EG Florida, California, etc. have no such p'tur.
Wolpoe's first Law of Mimetics
We often know WHAT to do without knowing the WHY we do it! :-)