Sergeant Bob said:
While indeed, if the Second Amendment were "safe", the NRA would be out of a job, the Second Amendment will never be "safe".
I think what is generally meant by "if the 2A was safe, the NRA would be out of business" is that if the Second Amendment was not perceived to be in great and impending peril, the business model of the NRA would have to scale down. Now, that might be a good thing for us firearms owners (if the NFA, GCA and AWB's were all defeated) but it would give gun owners a lot less incentive to give money to the NRA or any other gun rights organization. They would lose power, money and a large part of their size. No organization wants that.
For example, the ACLU has a better business model in that even though the rights they support are better protected and acknowledged, the issues they face involve mostly individual situations (one writer being censored as opposed to all writers or an entire publication) or a broad scope among multiple amendments and civil rights (patriot act, racism/sexism/etc...). There are tons of civil suits than can be lobbied within the scope of the ACLU's charter.
The NRA however, would not find itself in such a position as it is in if all it had to face were singular issues of disarmament because firearms rights are pretty well defined (even though not adhered to) and those areas that are not are defined within the scope of another aspect of the Bill of Rights or Constitution (private property rights or unreasonable search and seizure for example).
The ACLU has multiple dogs in the fight, the NRA is a one-trick pony as a lobbying and rights-based organization. They would have to further invest into sports/education/training and events, and they would have to compete in the market with some
very respectable outfits for that business.
The thing is, they've already done all of that...
And on the phone calls and mailings thing, it is very easy to get those to stop, just call the NRA. You're a member, they'll respect your wishes. If that's your only reason for leaving the NRA, it's not a very good one IMHO.