It's not pointless at all even if it happens. Only a stupid story would make the world so equal and fair that when the main characters get stronger, they only meet stronger and stronger adversaries as well. People need to be as strong as would be reasonable to expect due to the setting and circumstances. Impel Down is filled with seastone so in theory normal people should be able to handle things. However, they have a few tougher staffers looking after things as well. Any more than that might be improbable because only a person with a special sort of mindset would want to work and live in a place that dreary. Most would use their skills and abilities to find better jobs and opportunities elsewhere, for sure.
A character's worth doesn't lie solely on his fighting prowess. So, saying developing characters that won't last for two eps at minimum against monsters like Luffy is like saying they shouldn't have included Nami in the crew at all.