Dude, 70's is not 5 decades ago, who was your math teacher, are you catching a nap. Maybe you miss the part i said Dutch Disease, I think you should Google it to understand the meaning of the phrase. Everything you mention about Tanzania assets is within that phrase, so Google it.
Kenya your rushing to sale oil is because that is your survival, if you don't sale oil what else will you sale and give you same return as oil. But the only issues is, you've turn up to the oil party is just coming to an end. The prices are down, there more players, and the world is turning its back on oil.
As for the credit rating being import, who did we survive all these years with 7% growth real terms? how come we can still attract loans from other organization like OPEC, Arab league, Japan etc and in favorable rates and conditions? You go to Moody and Co. if you want to get yourself on bond market. Bilateral loans and trade is still performed by many governments including Kenya.