They can be destroyed very quickly and in fact, they often are. I’ve spent a lot of time using vehicles (which can easily be verified through stats trackers), and there were many situations where I died within 3–4 seconds after reaching a capture zone or on the way with getting 3-4 rpg's at once or one by one very fast.
That still doesn’t justify nerfing the RPG to the point where players can no longer reliably predict where the projectile will land, making the scope practically useless.
Instead, there are other ways to improve vehicle survivability. You could add thermal/night vision to vehicles, give IFVs a Trophy System, extend its active duration, and slightly adjust the cooldown/renewal time for that perk (extend by 3-5 seconds).
Tanks and armored vehicles have already been buffed overall (although IFVs currently feel worse than before, similar to the AA vehicle mentioned by MackTKau but in this case AA is practically useless against mid-skilled pilots). Because of that, there’s really no reason to keep the RPG in its current nerfed state.