infact it will lead to typical scenerios where prospective ICOers pay some «backshish» to get a rating of AAA+
Yes. But in decentralised model such ICOers could not buy good rating from every agency. So everyone can see incorrect behaviour and higher rating from bought agencies. Decentralised rating make rating transparent and inspectable.
The most practical way to deal with ICO scams is tying ICO to some utility product or service.
I don’t understand how it lead us to honest ICOs. Could you explain?