clergy were generally exempt from taxes, the peasantry and middle classes paid heavily. If a more comprehensive tax system had been instituted, maybe the lower and middle classes would not have been burdened with such high taxes and therefore may not have revolted.

These and other changes on the part of the monarchy and nobility may have prevented the French Revolution of 1789. We can never know for sure how things would have occurred differently, but it is possible that if the rulers of France had been more attuned and sympathetic to the hardships faced by the lower classes, the Revolution could have been avoided.

I might argue, however, that the status quo of pre-revolutionary France was unsustainable. If a revolution had not occurred in 1789, something likely would have changed at a later point. Remember that France was an absolute monarchy before the revolution. Today, (with the arguable exception of the Vatican) there are no absolute monarchies in Europe. It is unlikely that the French monarchy would have survived intact while the other monarchies of Europe adapted or fell over the next century and a half. How those changes would have occurred without the French Revolution in 1789, we will never know.

I think that any revolution could be seen as avoidable.  In historical hindsight, one can always play the hypothetical game of "What if."  This means that historians are always able to play around with "What if..." and be to craft a potential alternate history of a revolution.  In this light, I suppose that a historical revisionist could make the argument that the French Revolution was avoidable.  Had the monarchy been more receptive to the needs of the body politic, perhaps the revolution could have been avoided.  Had Louis XVI not raised taxes to such a high level, perhaps discontent could have been shelved for a bit.  Had the aristocracy had better insight into letting the new working class into the economic system of the time, perhaps there might have been some deferring of their anger at being excluded.  Yet, the reality is that the Status Quo proceeded with such a fervent authenticity in its own political and social notion of the good that reflection seemed impossible.  The absolutism with which the ruling monarchy and aristocratic class proceeded was so dauntingly abrasive that it left no room for potential negotiation.  The French monarchy seemed so convinced of its own certainty in ruling and could not conceive of popular uprising so successful as to destabilize it that the revolution almost seemed inevitable in this regard and not avoidable.