The problem with your scenario is that, everybody know or at least had to assume that this would only delay unification not prevent it forever, so they had to factor in the long-term consequences of trying to force Germany by any means short of war to stay apart.
I am sure France would have loved for Britain, Italy and the Netherlands to poison their future relationship with Germany to delay reunification. And Italy would have loved for France, Britain and the Netherlandsto poison their future relationship with Germany to delay reunification. And so on. But no one would have been willing to be that first one to play the bad cop.
They were not playing a strategy game with the objective of delaying German unification as long as possible. They had to look at the long haul.
If the scenario you describe had come to pass then, after say delaying unification by 10 years the price Germany's "EU partners"

would have paid for that would be:
1. A Germany with much less of an ideological commitment to the "European Dream" and as willing to play hardball as anyone else instead.
2. A Germany that's the best friend of the U.S. 2nd or not perhaps not even even that compared to the UK.
3. A Germany that would look to befriend Eastern Europe to counterbalance it's frenemies in Western and Central Europe and almost certainly succeed, since those also are much more pro-US and anti-Russia then the rest of the EU.
10 years after that and you've basically created a de-facto Mitteleuropa. The Germany-less in TTL Eurozone would also lack any other of the Northern members being de-facto a Latin currency union with no more stability than somewhere between the Franc and the Lira. Wheras the Deutschmark has become the unofficial 2nd currency in much of Eastern Europe, giving Germany a de-facto currency union who's fellow members don't have any seats at the Central Bank nor can demand bailouts by threatening to bring it down.
Ironically history books in Britain and France will probably congratulate Thatcher and Mitterrand for their foresight about a resurgent Germany and how they wisely tried to prevent it.