Not taboo but it was really stupid thing to say, especially since you admitted you have no idea how this industry works.
In AKB even unpopular theater girls (aka the ones who just work in theater and are not on TV, in CMs, etc.) can earn enough money to live by themselves and not being supported by parents (like 99% of Korean trainees).
Korean girls after P48's group disband will debut in new groups from their agencies but if AKB girls graduate now what will they do later...? They won't be able to survive on their own. Not in Korea not in Japan. That's why it was ridiculous to say "they should graduate".
They joined P48 to boost their popularity because being popular in AKB opens many doors for idols. The reason why many older girls don't graduate is because without AKB name there won't be many jobs for them. All SUPER POPULAR AKB members had to start basically from 0 after graduation. As singers, actresses, models or talents. Even Maeda Atsuko and Oshima Yuko are playing supporting roles in dramas now because they have to build their name once again, not as idols but as actresses.
P48 group will be there ONLY for two and half years. For AKB girls this amount of time is not that long if popular ones can be in group for over 10 years. They can lessen AKB related schedules like theater performances, AKBingo, handshakes, smaller events etc. but all of them will participate in SSK next year (except for Sakura who already resigned), important performances on TV, year end shows, etc.
And as someone said. They will just be Kennin members of both.
Traveling from Nagoya (SKE48) to Tokyo (AKB48) by Shinkansen takes about 100 minutes. From Fukuoka (HKT48) 2 hours by flight. From Niigata (NGT48) and Osaka (NMB48) it's even longer iirc but flight from Tokyo to Seoul takes only 2h 40min. Japanese idols are used to traveling within Japan. In Korea most activities are in Seoul, sometimes Busan.