Always_Ask_Questions,
“We insist on eating hot food to show we are not tzdukim. Maybe same applies to eruv?”
Being a modeh beruv is one of the issues that the tzdukim did not accept. I will admit that what we can infer from this is up for debate. However, I believe that these disagreements do descend into being an eino modeh beruv. I would argue, that those who vehemently oppose today’s city eruvin, are eino modeh shtufei mavaos. City eruvin are an outgrowth of the halachos of shtufei mavaos, and is a separate mitzvah classified in the Shulchan Aruch (O.C. 395:1).
“One interesting argument I see in this discussion – if some shita is constructed in a way that invalidates all eruvin (or, if I can expand – invalidates what was normal practice before) – then it is apikoirosus, not chumras. Thus, if there is a kosher eruv, one must carry. There is probably a loner list of activities that one must similary do because of people asurim what used to be matir. I’ll leave it to others to suggest them.”
I would believe that when it comes to chumros we should look to see consistency (I know that some argue otherwise, but with eruvin this should be the benchmark, because of the eino modeh issue). Even more so, in hilchos reshuyos and eruvin, since all criteria have to be met for the area to be classified as a reshus harabbim, even if we were to employ a shitas yachid regarding reshus harabbim that would then disqualify the eruv based on only one criterion, the other conditions would not be met and an eruv would be permissible l’chatchilah. Consequently, to invalidate an eruv, one would have to selectively choose from disparate shitos yachidos ― which in many cases are contradictory ― and that is an unjustifiable approach to halachah. The reality is that if someone learns hilchos reshuyos and eruvin with an open mind, he would realize that since it is almost impossible to meet all the criteria of a reshus harabbim, creating an eruv l’chatchilah is a real possibility.