Gaon, I didn’t say Rabbeinu Tam tefillin is the only case, that just an example of where it’s spelled out. The point is, if there is a concern of being machmir leading to gaavah, it will be spelled out explicitly, which it is not in the case of eruvim. Why WOULD you apply it to a case where it does not say so explicitly?
youdontsay: Is what you’re saying actually mainstream? That the Mishnah Berurah was wrong about how most rishonim held and that were he alive today he would correct the “mistake”? Do you have a source that says this?