The problem is that at that point the USA, Canada, France, Italy would be obligated to defend Russia if it was attacked by sending in its forces. This is already the problem we face in the Baltic States. In theory you can say you will go to war if Estonia is attacked. But if you are Italy or Canada how popular is a war in Estonia going to be if your citizens are dying? There would be a lot of pressure to pull out.

Once the guarantee of NATO becomes a maybe then you start having a lot of incentives for the members to make their own agreements that they can depend on. Then you would be back to regional alliances and from there rivalries. Germany's security agreement partners may not agree with Turkey's security agreement partners about what to do in Syria. Part of what made NATO work was the common interest and values of the members. As you spread it you loose that cohesion.