It is my understanding that shah sahib was addressing the issue of tafdil and ijma, and that because there was no ijma in the times of sahaba on the issue and they differed about it hence agreement after disagreement in a previous age does not authorise the new age to establish an ijma. And al-Sarakhsi's quote on that has been provided below. However, being a wise man, you have avoided the crux of the argument about ijma on tafdil and ijma about Parents (alayhuma al-salaam). That is what you should have refuted ... Anyway, ya shaykh, there are notable usuli imams who declared unequivocally that epistemologically it is not possible to ascertain an ijma other than that of the sahaba. for example, here is al-Razi's conclusion in his al-mahsul: والإنصاف أنه لا طريق لنا إلى معرفته إلا فى زمان الصحابة the correct position is that we have no way of knowing ijma upon an issue except in the times of the sahaba.