this entire discussion smacks of the very xtian concept “let he who is without sin cast the 1st stone”.
We had it first and they stole it from us! See Bava Basra 15B.
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To the OP: The reason people fight back is because it affects us personally. Like Oomis said in the Tefilin thread, once someone starts a Chumrah, it becomes mandatory or your child will not be able to get married. So you have to fight just to make most of the Klal’s current position acceptable.
On the other side, if a foreign idea is seeping into Yiddishkeit (the classic example is Toeivah), then we have to fight the intrusion so that our children don’t grow up thinking that it is acceptable.