Tosafos 28b explains that the stipulation made on shuls that one should be able to derive personal benefit from them only applies to shuls that are outside the land of Israel and only applies after they have been destroyed. Tosafos explains that the stipulation doesn't work for shuls in E.Y. bec. there kedusha will always remain. But for shuls outside E.Y. the kedusha will go away (therefore, when they are no longer serving as a shul i.e. after they have been destroyed, there is no kedusha that remains). Magen Avrohom 151:15 asks that we say on 29a that the shul will be transplanted to E.Y. so the kedusha should linger similar to the shuls already in E.Y. He answers that any shul destroyed before the bi'as go'el will not be transplanted in E.Y. The Korban Nesanel #3 answers that the shuls outside of E.Y. will become part of the ground in E.Y. but they will not be shuls in E.Y. so they will only have kedushas E.Y. not kedushas Beis Hakneses.
I may be missing something, but when I learned Tosafos I understood the gemara to be supporting their explanation (opposite of Magen Avrohom's question). The shuls outside of E.Y. will be moved to E.Y., meaning the ground will be left void of a shul hence void of kedusha, therefore the spot on which the shul stands now has no kedusha (once the shul is destroyed).
I may be missing something, but when I learned Tosafos I understood the gemara to be supporting their explanation (opposite of Magen Avrohom's question). The shuls outside of E.Y. will be moved to E.Y., meaning the ground will be left void of a shul hence void of kedusha, therefore the spot on which the shul stands now has no kedusha (once the shul is destroyed).
