Given that it takes a long time to bring a street up to standard (budgeting, design, contracting, and constructing), that would probably be 10-20 years at an optimistic estimate to get every street up. In that time, under your proposal, the roads would become undrivable, and therefore:

• Emergency vehicles would be unable to operate. Thousands die.
• Traffic increases exponentially as the usable roads become increasingly infrequent and commuters flock to the few good ones. The above problem is made worse; gas usage increases dramatically as more and more cars sit idle for hours a day.
• Highway safety plummets. Thousands die in avoidable crashes.
• Roads become impassible to trucks. Deliveries of food and goods grind to a halt. Starvation, food riots, economic collapse follow.

I'm all for increasing walkability and bikability; I'm fortunate enough to live in a city that is both, and it's great. Proposals like this, however, do nothing but make it look like the movement is a bunch of "fuck cars" knee-jerkers who know nothing about infrastructure and can thus be safely disregarded.