For Avon wasnt about the money, it was more about the respect, power and all the stuff that comes along with it, Avon had a code and operated with for the most part some kind of honor. Stringer was the opposite, all about the money, which is what did him in, in the end, on top of that it turns out, breh wasnt as smart as he thought he was seeing as how he ended up getting extorted by some crooked politicians.
As far as him losing power in the streets, obviously he lost his largest base of operations when they knocked down the towers, also his connect in New York ended up getting caught up and he couldnt deal with Avon anymore, so his crew didnt have the product to keep the territory, plus Avon himself couldnt stay out of legal trouble due to the cops being on top of him at all times. Marlo got severely lucky son, them nikkas had grenades when they got raided

not that marlo wasnt a capable smart guy but luck and circumstance had something to do with him gaining power
At the end of the day, Avon just ended up getting dealt a bad hand, and if you look at everyone else in the series, he came out pretty well in the end, seeing as how today in 2k14 he would've already been out of jail for awhile now