The Concert was indeed an interesting time, but the problem with making it last much longer is that it was by the 1840s based partly on illusions: the various balances of power had to change in the space of decades.
If we look at the conflicts, armed or not, that blew up in the space of 1848-1853 (which was the starting gun for the "struggle for mastery" that lasted until Bismarck won and started working to preserve the new status-quo) we see that many of them result from these changes:
- The Austro-Prussian struggle, which began in the revolutionary context (even after both sides had crushed their respective liberal radicals, they still engaged in a contest for physical influence that ended with Prussia's premature ambitions being humbled at Olomouc), basically happened because Metternich had wanted Prussia to be Austria's sidekick in dominating Germany, but hadn't know about all that iron and coal under it. Mushrooming economic importance gave Prussia political influence via the Zollverein, and so it started to aspire to being an independent great power with an independent foreign policy, and then to dominating northern Germany. It's hard to prevent this without a majorly differant 1815 settlement of Germany.
- Italy was a bomb waiting to go off. When you combine a popular revolutionary cause with the state interests of the peninsula's biggest and most independently powerful state (Sardinia) and the fairly consistent goals of French and British foreign policy, you've got a recipe for trouble. Italy can be relied on to go up as soon as the Austrians are in difficulties anywhere.
- The Russo-British confrontation at the straits was another one which had been a long time brewing. Russia had, after all, been on a pretty continuous winning streak against the Ottomans for upwards of 100 years (since the 1730s), so it was natural that what they and us saw as our respective spheres of influence would bump. A large part of Metternich's system had been convincing Russia to exercise restraint in the Near East out of fear of revolution in Europe, specifically, Poland; but in 1848, of course, Poland and especially Congress Poland utterly failed to detonate.
That's the key thing, really: the Concert asked every power to surrender its own cynical interests on behalf of the anti-revolutionary cause, which none of them were ever actually going to do (Britain had basically gone off in a huff as soon as a large infringement of our own interests, the reconquest of Spanish America, was floated). 1848, when lots of revolutions failed or turned out not to be quite in the mould of 1795 (the Second Republic, and the Second Empire, were probably less aggressive than the July Monarchy: the Russians certainly thought so, since the Orleanist parliament had held an annual grumble-about-Poland session), was just a conveniant signal for everyone to start fending for himself, 18th C style.
I think the decay of anti-revolutionary unity of purpose is difficult to prevent, as is its physical counterpart, the return of France as a "normal" European power.
That's not to say either that an "1848" is inevitable or that European diplomacy couldn't have unfolded in a dramatically different way with the right PoDs. The Crimean War, after all, nobody wanted except maybe the Ottomans: it was the result of one damn thing happening after another.
What 1848 was was "nearly every brewing crisis goes off at once except the ones (Belgium, Russian Poland) that had already gone off". If we could avert some of these crises and spread the others out, there's no 1848* as such. I think the domestic free-for-all the befell Austria wasn't inevitable and Metternich, had he been given a freer reign, had ideas that could have prevented it and helped Austria reform itself with only more sporadic outbreaks like Galicia in 1846. No domestic Austrian crisis means an outright war for Italy is unlikely to come so soon. Belgium might be crushed if you change the timing of its revolution relative to Poland, which would be a big dent in the recovery of French confidance and influence.
By sidestepping all these outbreaks, one might end up with something along the lines of "The French have a revolution (again) - After some grumbling, nothing is done about it (again) - The powers of Europe continue to make thumbs at each-other during Congresses and manouvre to better one-another, but nobody actually declares a free-for-all."
That's something, at any rate.