No fandom no matter how loyal they are can withstand a 18 month hiatus, if your counter to that is "But Blackpink?" you're wrong, they might not release music as a group but they're always active in the public eye, last year they had 2 members release solo's and 1 member doing a drama+ tons of endorsement activities , that's not the hiatus bts will be facing, bts will have to be radio silent for 18 months.
I'm not saying BTS will flop or anything (even if they lose half of their fandom it will still eclipse any other group by a mile) but their momentum will be disrupted for sure.
The best option for them (if the are not exempted) is to let the hyung like enlist first and makne like promote since they have still at least 5 years till they have to enlist.
But then again, how is it fair to j-hope and RM since they basically have to enlist 2 years early, it all depends on their loyalty to the bts brand.