As each Doomed Weapon costs 55 scrolls and a new weapon is released every time the Doom event is played, it is virtually impossible to collect enough scrolls to fully upgrade the weapons.
Unlike regular weapons, there are other means to obtain ingots, but with doomed weapons we can only accumulate scrolls during the tower of doom event, making it impossible to keep up.
Please can we have other ways of obtaining scrolls please.
I dont see why we need 6 types of scrolls.
Not quite, but I get the point and totally agree.
Here’s some math anyways.
You’ll get 25 scrolls for defeating the first 25 dooms, plus 7 for buying the weapon for 500 Gems in the first place.
That will leave you at 32 ‘only’ missing an additional 23 scrolls to be obtained.
Checking the shop rewards/prices, you’d have to buy Tier VII three times for a total of 55 scrolls which ‘only’ costs 2850 Gems total - every 4 weeks.
Sound about right to me. 
Edit/
Just realized my mathematic skills were wrong. Corrected the above equation.
They want you to buy Tier 7, 3 times and complete the first 25 floors of ToD for each ToD.
This is working as intended.
I suppose when the next cycle of the same-coloured Doomed weapons comes around, they’ll be expecting you to choose between fully upgrading your first one and partially upgrading the second (assuming you didn’t already max out the first).
Who knows what’ll happen after then? In a way, it kind of makes sense that it might take a while to fully upgrade such powerful weapons, but once every 6 months isn’t an easy timeframe to work around.