And why would a really low TH level care about an extra loot bonus for leveling up? So what if a TH4 gets a huge loot bonus for reaching level 50, or level 75, or level 100, or level 125, or whatever? What are they going to do with it? They should be several TH levels higher, where the loot bonus would actually buy something useful.
I think this is a great idea. The loot bonus for leveling up doesn't need to be a lot. Maybe 1k-2k of each resource per level? So reach level 67, get 67k (or 134k) of each resource? If you're TH7 and up, earn 5-10 DE per level?
It's not enough to buy a major upgrade, unless you're way behind where you should be for a given level. But it's enough to help out, like a league bonus.
At 2k per level, by the time you get to level 75, you would have earned less than 6M of each resource, cumulatively. That's not a game changer, but it would help out.
The XP bar is a lot more indicative of how good a player actually is for the most part. A Bronze III you would expect to be a noob, right? Nope, there's a lot of Bronze III's that are high level TH8/9/10s because they're farming and cup dropping.
If you're a level 87 there's a 99% chance that you're experienced and know what you're doing, but there's a 50/50 chance that you're in Gold/Crystal.. However, if you're a level 10, there's a 99% chance that you're inexperienced but there's a 99.99% chance you're in Bronze/Silver 3.
I hope I got my point across.