None of these reasons seems strong enough to make this a "terrible idea" really.
First of all, someone who is not a drug addict, but merely trying to take naltrexone for PCT or as an anti-aging drug will likely not need the dose that is considered "therapeutic" for an addict. Instead a significantly smaller dose will likely work. The factor of 5 between therapeutic and toxic, will likely become 10 if not higher.
Also, what if naloxone is injectable? There is naltrexone and people can just use naltrexone instead...
and "you cannot take opoids for a while before starting this and also messes with the feeling of alcohol ingestion". Yes, you cannot take opioids, but if you do, you will simply render them ineffective. In fact this is what the drug is used for, no? To make addicts not enjoy opioids... And if someone doesn't have the discipline not to drink while on PCT, well they should not use this, plain and simple.
To read more about what great things naltrexone can do, check out Patrick Arnold's blog:
http://patrickarnoldblog.com/
look at his post titled: Interesting anti aging drug (part 3)
I think naltrexone can be very effective both during cycle and PCT.