This emphasis on sustainability incorporates aspects from both movements. Lewis and others are quite right to note that, for the Founders, private and public responsibility play an important part of maintaining a free society. If those at the economic and political top believe in simply aggregating more and more wealth and power to themselves without any compassion or public virtue, we would not simply risk having a tiny super-elite and an ocean of the poor. We could also witness the free market and democratic process being corrupted, as this super-elite uses regulation, taxation, and subsidies to grant more power to itself. Moreover, the poor and weak, stymied at every turn, could easily look for revolutionary means to overthrow their selfish overlords. See the French Revolution for more details.
Empathy, compassion, and a sense of public spiritedness are great helpmates in maintaining a republic. The "compassionate conservatism" preached by George W. Bush was very laudable in its motivations, but the practice of this "compassionate conservatism" often fell short. However good its intentions, "compassionate conservatism" often failed to treat the driving causes of economic decline in the United States. The real estate and financial bubbles of the Bush presidency did not compensate for the stagnating wages and slowing economic growth of the 2000s; if anything, they ended up compounding inequality and further undermining the economic vitality of the nation. It's not enough for the government to cut a check after the fact in order to compensate for a lack of economic opportunity. Instead, opportunity must grow organically within an economy (though there are certain things government can do to act as fertilizer for this opportunity). A popular prosperity is likely far better than redistributive government subsidies passed to compensate for the stagnation of the economic middle.
Reformist arguments that conservatives should focus on practical improvements for government have a considerable Reaganite pedigree. After all, throughout his two terms, Ronald Reagan worked on fixing---not destroying---the post-New Deal social compact: compared to the setbacks of the Carter administration, the successes of Reagan proved that the federal government could still work, and the Social Security reforms championed by Reagan made the program sustainable for decades into the future. Sustainable conservatism recognizes that severe dysfunction challenges a broad public faith in government and that the maintenance of a free, civil society seems to involve some kind of belief in government. By focusing on practical, targeted improvements to government policies, conservatives can prod public policy in the right direction, and reforms to ensure that programs are administered as effectively as possible can gratify the ideals of both small government and good government.
Sustainable conservatism would seek to expand opportunity and to ensure that a broad range of Americans enjoy the fruits of prosperity. The tactics of a sustainable conservatism are varied, but here are some things we might consider: financial reform that ends the era of "Too Big to Fail" and ensures a market-oriented and transparent financial sector; tax reform that encourages work and strong family formation; an approach to immigration that provides opportunity for recent immigrants and native-born workers alike and that does not subsidize a race to the bottom in wages; market-oriented health-care reform that enables individuals to gain affordable health-care without breaking the public bank; policies that encourage industrial and energy development; and reforms to make federal entitlements sustainable (especially Medicare). These sorts of policies would make government finances more sustainable and help foment a prosperity that would lessen the need for big government.