It was designed this good long in the past, before any other thing RuneScape gold. At the time , it was the absolute hardest content you could do especially given the gear they had at the time. It was intended to be a powerful flex given that it came from an area that people thought would forever be the most difficult task in RS. Today, with the enthralling spells, bows, blowpipes and brews, high healing foods, incredible armor, and prayer switching/flicking mechanics as a fundamental aspect of PvM, it's nowhere near as hard as it was in the past, so it's lost the bit of prestige. Although trivial to a lot individuals, the cape is an obligatory rite of passage for any new player. I feel like the ironman game kind of brought it back to its origins due to their limited access to gear and supplies when they get enough to where they are able to try it.
They did not "screw themself" by creating the fire cape as strong as it is. The battle caves slowly moved from the most challenging to easy content as gear materials, supplies, and the combat mechanics' power grew. When they made the fire cape, which was the ultimate reward from the most difficult content. It had to be effective. I believe it does.
The thing I'm saying is that in creating the fire cape as great as it is they didn't have a plan for the possibility that it might not be the best anymore without creating super-powered caps. Like how the myths cape is difficult to obtain nowadays and yet they can't enhance it, without creating fight caves that are useless to use. I understand why they created it the way they did however, they could have future-proofed it more by giving it +2 or +3 strength and prayer.
Take a look back OSRS gold at the bosses back when OSRS first came out (using the wiki to calculate dropping rates, and deliberatively forecasting kill times based on memory) -DKS, each having three uniques of 1/128 and the battle lasts for one or two minutes. 
|
.jpg){kind=small}
