Respectfully, you have no clue what you're talking about. Crims do NOT keep their wellness low because it allows them to die faster, this is just an outrageous take/accusation. Low wellness is literally just useless, you run slower, die quicker (PVP) and is generally a pain in the ass. First thing I do after dying is go to Bennys or to a shop to get food and water to remove my death effect and increase my wellness.
They do provide a massive advantage against those not using them, notably steroids as they give you an extra 25% movement boost which is in theory unobtainable by LEO. However, heroin acts as a 40% AP and the highest attainable AP being 100AP for crims means that this essentially equates to 140AP whereas I'd like to say 100-150AP is easily accessible to the majority of LEOs especially considering that there are significantly more LEOs than crims in one specific gang. Additionally, methadone is not some easy fix forever as it only reduces the effects (visually) for some time before you get absolutely flashbanged by heroin alone, nevermind anything else you take, although I don't think that this needs to be changed.