2 years isn't short time. Some of BTS members might be active while the others are in military. Kpop fans from Army might protect the boys by staying be fans. The problem is media and achievements will not wait. Those 2 would definitely search for replacement. For example, Monsta X and GOT7 start to gain attention in just 1 month of BTS being in hiatus. With constant media hyping for more than 2 years, there's chance that those 2 will grow a lot bigger and fill the void. BTS themselves start gaining huge attention starting 2016 when BB enlisted.