> like containing Russia without having to rely on the whims of another country's politics

That's true, but at the same time it was probably already the case before invasion of Ukraine, and it is definitely the case now.

The main issue is political fragmentation: would Paris and Berlin risk lives of French and German people (soldiers and civilians due to retaliation) to save Vilnius?

But if the answer is true (as obligated by the Treaty of Maastricht, independently of NATO) then Russia stands no chance with conventional weapons against the whole Western Europe, the balance of military, demographic and industrial power is ridiculously lopsided (involving nuclear weapons would also raise the same political question about the French willingness to nuke Russia in retaliation to Russia nuking Poland but if the answer is yes, Russia cannot win a nuclear war either (which everyone would lose)).

The answer is always going to be "maybe", but hopefully enough of a maybe to deter hostile actions. That puts everything in an uncomfortable state of uncertainty.

> The main issue is political fragmentation: would Paris and Berlin risk lives of French and German people (soldiers and civilians due to retaliation) to save Vilnius?

This is a wrong question. If one day Russia feels brave enough to attack any NATO country, the right question to ask is, "Do we want to fight this war on someone else's soil or on ours?". This is the reason why Europe is so focused on helping Ukraine BTW.