Aaq, bear in mind we’re dealing with a deoraysoh…also we’re not talking about practices of chutz laaretz, where the minhag of being maikil is established. We’re talking about a pirtzas geder in eretz yisroel based on “mitztayer that you’re not mitztayer” garbled reasoning. To say that everyone nowadays is an istinis regarding aveilus, is not so shver because we’re supposed to be naikil in aveilus and it’s likewise a derabonon… it’s also a question of veing very uncomfortable.
Here as we can see from the millions of jews who sleep in a sukkah in eretz yisroel, they are not istinis in this regard..
If one regards himself as an istinis, as the aruch hashulchan says he must be that way in every respect, not just when it’s convenient. Bochurim who go a whole week without showering(this happens in all communities which have bochurim, chabad included…not all, but I’d say 15% in Litvishe and Chasidishe yeshivos) are not istinisim, but they also don’t sleep in a sukkah in neo-chabad…. that’s a clear contradiction