This sounds like a valid concern, but it seems to me, that this is more a problem of balance, i.e. what "x Blocks" means exactly. You could probably make "X" dynamic, maybe by adjusting it every "Y" blocks, but that might open a whole new can of worms
Another problem, as mentioned before, is that it precludes people with 0 XEL from becoming work miners to get XEL. If you want to mine XEL you'd first have to buy XEL, which is less than ideal from the miner's perspective.