As a UIM with 200mill in most of my skills, I think I can provide some perspective on this idea since I didn't know about the nightmare death bank mechanic for the majority of my grinds. This game mode is specifically designed for me. But I'm against it
Yes, the XP rates are way too high on a regular UIM. I'd love to play one with lower rates.
HOWEVER,
The way this game mode is suggested would never work. Without dying, UIM are only able to take things out of their looting bag by destroying it in the wilderness. The second you destroy a looting bag in the wilderness, all of your items are available to be seen by anyone. You'd have to pick them all up immediately. That means you need 28 empty inventory slots if you have a full looting bag. This will restrict you much more than even on OSRS, where you can actually death pile. Not to mention looting bags on all wildy monsters outside of bosses is a 1/30 drop so to even rebag stuff you'd be risking your HC status every time just to rebag. In OSRS a looting bag drop is like 1/4 which is much quicker to get. So if at any point you have 28 looting bag items + 1 extra item in your inventory. You are stuck. You'd have to destroy your looting bag in the wilderness to access your items, then quickly kill 30 low level monsters (if you go on rate but you can go dry too) in hopes of getting another looting bag before the item on the ground despawns or someone else picks it up. I'm sorry, but that's unnecessarily hectic
Having items on the ground for 1 hour is such a big part of the game mode. I wouldn't be against a HCUIM that can't use the nightmare deathbank with lower XP rates. But deathpiling mechanics MUST be there.