If a person forgot to say Ratzei and they already began the next blessing, however they know with total certainly certainty that they will be eating another bread meal, why must they repeat benching instead of allowing the next meal be considered one of the obligatory meals of Shabbos?
Even if he knows that he will be eating another bread meal he still has to bentch again, and it isn’t as if we have to option to choose which will be the “obligatory meals”. This is because when the person ate the bread meal, he already fulfilled his obligation to eat a meal, and therefore it is on this meal that he has to say ritzei, and if he doesn’t he has to go back. ( It is the meal that makes the obligation to bentch correctly, not the bentching that dictates what the meal is.)
Besides this, the halacha is that we have to wash and bentch even for shalos seudos, (O:CH 295-4,5, M:B Ibid 23), and even those opinions that one can be yotza with cake of fruits, that is only that b’dieved he can still be yotza, but the main obligation is to wash and bentch. Therefore every shabbos he is going to eat a third meal, but the halacha still says that he has to bentch again. What the Shichan Aruch says 188-8, that if one forgot to say Ritzei during shalosh seudos tat he doesn’t have to bentch again, is only since there are opinions that one of yotza without bread, therefore his obligation to bentch again is questionable, therefore he doesn’t bentch again, but he essential obligation to eat bread for shalos seudos still exists. Even though we have to wash for shalos seudos, nevertheless the Shulchan Aruch rules that we have to bentch again if we forgot to say ritzei.
O:CH 295-4,5, M:B Ibid 23, O:C 188-8.