Article 1 of the NATO Treaty reads:
The Parties undertake, as set forth in the Charter of the United Nations, to settle any international dispute in which they may be involved by peaceful means in such a manner that international peace and security and justice are not endangered, and to refrain in their international relations from the threat or use of force in any manner inconsistent with the purposes of the United Nations.
Since certain NATO members with quite some military capability have excused themselves from the requirement "to refrain from the threat or use of force" in that Article lately, while certain others refuse to dump international law, it makes sense that the law-abiding nations are looking into other forms of military integration to replace NATO.
This is necessary for reasons of German constitutional law.
The German Federal Constitutional Court found on November 22, 2001:
"The elements of the wording of Article 24.2 of the Basic Law preclude, however, the Federal Republic of Germany’s participation in a mutual collective system of military security that does not serve to maintain peace. The transformation of a system that originally fulfils the requirements of Article 24.2 of the Basic Law into a system that no longer serves the maintenance of the peace, or even prepares wars of aggression, is constitutionally prohibited and can therefore not be covered by the content of the Consent Act that was adopted on the basis of Article 59.2(1) and Article 24.2 of the Basic Law."
Since important NATO members now have a policy of waging illegal wars of aggression, Germany would violate the constitution by staying in NATO. Looking for alternatives becomes a logical and necessary step.
Even if one does not follow the above opinion of the Federal Constitutional Court, it does not make much sense to pretend that America and the UK still qualify as allies of countries who respect international law. NATO can't work anyway if its most basic foundation in article 1 is not recognized any more universally by all members.