[NATO is an alliance without a purpose other than to enrich arms dealers, a relic that should be discarded yesterday.]
They can increase their spending whether or not NATO exists.
I don’t feel comfortable with advising any country that they must increase their defense spending. Who exactly is the threat? But I DO agree with Trump that given the premise that NATO is neccessary and good (a premise I utterly reject) then the members should pony up more and be more responsible for their own protection. But overall I see the US and other governments as basically criminal money-laundering enterprises and arms contracts are a vital cog in that machine. As a youth I was quite the hawk — more was always better. But now I see the bulk of defense spending as the means by which politicians get and stay elected (among many such means).