Burke ever held, and held rightly, that it can seldom be right… to sacrifice a present benefit for a doubtful advantage in the future… It is not wise to look too far ahead; our powers of prediction are slight, our command over results infinitesimal. It is therefore the happiness of our own contemporaries that is our main concern; we should be very chary of sacrificing large numbers of people for the sake of a contingent end, however advantageous that may appear… We can never know enough to make the chance worth taking… There is this further consideration that is often in need of emphasis: it is not sufficient that the state of affairs which we seek to promote should be better than the state of affairs which preceded it; it must be sufficiently better to make up for the evils of the transition.
— John Maynard Keynes, Burke’s Timidity on Embarking on War, 1904
(via Brad DeLong)
Note, however, that when the status quo has become unendurable, it is the conservatives, not the revolutionaries, who are intent on "sacrificing a present benefit," and not for even a "doubtful advantage": if we keep things the same, how could conservatives expect anything but a repeat of the present misery?
Keynes and Burke's advice is so obviously true that one must wonder why they offer it. I'm not an historian, but I've read a couple of books, and when I look at the revolutions in history, I don't see any revolution that sacrifices a present benefit for a doubtful future advantage. Revolutions have usually occurred because the status quo had become unendurable, and there was no present benefit left to sacrifice. Alternatively, (as in the bourgeois revolutions) the revolutionary class had made its benefits manifest and present before the revolution; the revolution occurred only to recognize the already existing real economic and political power with nominal state power. If I'm wrong, let me know, but I just don't see that we ever, as a society, have sacrificed a prosperous present for a conjectured utopia.
The closest we can come to a society ignoring Burke's and Keynes' advice are those two sacrosanct* conflicts: the American Revolution and Civil War. British rule of the American colonies was not catastrophic; independence was about as dubious a benefit as one could imagine, and it nearly failed. Slavery was hardly unendurable (for white people) — it had been legally tolerated for generations — and to even begin to obtain basic participation in civil society, black people had to wait a century after the conflict. Even the benefits of preserving the Union were at best dubious; the nationalist sentiment of the Confederacy still simmers today.
*Sacrosanct to Americans, at least
Why then do Burke and Keynes appear to be stating the obvious with great profundity? I have yet to read much Burke (and, again, I'm not an historian), but it seems clear that Burke had a political agenda considerably more substantive than simplistic caution and cost-benefit analysis. A key component of Burke's political philosophy appears to be that the legitimacy of the state is tradition. It is not that we must proceed cautiously and ensure that the benefit of a radical change in tradition will be worth the cost; a radical change in tradition is inherently evil, because tradition itself is the only political good. Burke's conception of tradition appears to make any cost-benefit analysis irrelevant.
Of course, Burke's conception of what constitutes legitimate "tradition" appears somewhat elastic*. Even the radical notion of democracy can — by appeal to the centuries-dead Athenian democracy and Roman Republic — be given the sanctity of tradition. Burke must not only ground legitimacy in tradition, itself problematic, but also give us a methodology as to which traditions establish legitimacy.
*Keep in mind that I'm relying on secondary and tertiary sources.
Keynes is at least on somewhat firmer ground, as he was probably the guiding intellectual force behind the successful reform of laissez faire capitalism into state-managed capitalism. Burke has no such excuse: the conditions in France that led to the French Revolution were truly unendurable. When tradition catastrophically fails, there is no sense in wondering whether it should be abandoned; we can only speculation on how it will actually be abandoned. Revolution is dangerous, revolution can easily go awry, and thus revolution cannot be undertaken lightly. But the French did not take revolution lightly. Tradition had utterly failed them; their only alternative to revolution was slavery in the guise of tradition. Burke might have argued for a better revolution, but to use the French to condemn revolution and laud tradition, to support the catastrophically inept French monarchy, Burke might as well argue against glaciers.
While he is not inexcusably obtuse like Burke, DeLong is not, like Keynes, justifiably optimistic. The professional-managerial middle-class reforms that Keynes midwifed to save capitalism have, after three generations, decisively failed. We cannot simply make the same reforms that we made in the early 20th century. Even if we could make them again, even if we could take the rentier ruling class again by surprise, they didn't last then, and we have no reason to expect them to do any better tomorrow.
Burke might be correct in a rhetorical sense: no matter how radically we transform our institutions, we might well be best served to give our new society the imprimatur of tradition. We need not lie; we need only to be selective. But to say we must be cautious is too obvious; to be actually meaningful, this supposed "caution" can be nothing but an attempt to justify the unendurable failures of the status quo with the bugaboo of the fear of change.