The change need not be dramatic; it need not apply to all oil revenues at once. For example only revenues from new fields could go into the independent fund while existing revenues to the government are maintained. Services therefore won’t need to undergo contraction.  The impact on politics would also be minor; people elect those who get things done not those who blame others for their problems. Moreover all of the politicians will have the same constraint of a lack of funds so no single party will have an unfair advantage.