I think it became overly dogmatic in some aspects and too "rigid". The partnership in the Treaty of Friendship, Cooperation, and Mutual Assistance became more and more top-down instead of equal, which is in itself a contradiction of internationalist marxist policies that were advocated for. In response to NATO, the system became more of an extension to soviet bureaucracy, which angered many of the working class in the other pact states, because it felt like or "resembled" a system which felt too close to imperialist rulership with peripheral countries. Also, the COMECON failed to achieve economical convergence and rather deepened devolopmental gaps between the member states (for example compare Romania's economy to GDR and you will see what I mean). Some people also complain about the bureaucracy becoming it's own class but I think it's bs, I am not a leftcom. Although I think some were becoming too greedy in the way of wanting to own things a common worker could almost never own. This also stifled resentment.

So, to conclude I would say the Warsaw Pact did not fail because the idea of collective socialist defense was invalid. The threat of NATO was very real. It failed because its actual practice betrayed the very socialist and internationalist principles it claimed to uphold. It became a mechanism for bureaucratic control, masking deepening economic inequalities and national tensions. Its dissolution was the political-military consequence of the broader crisis of a specific, historically-formed model of socialism, one marked by bureaucratic deformation, economic stagnation, and a negation of genuine workers' democracy and national self-determination. Therefore, its end is seen not as a refutation of Marxism, but as the necessary collapse of a structure that had deviated from core Marxist principles.