I think I can contribute some helpful, and balanced information.
First, you are correct, ALL breeds were originally crosses at some point in time -- it depends where in the time continuum you pound in the marker -- does 200 years since the crosses were used define a 'pure bred'?100 years? 50 years? Less?The American Mastiff has now been around for closing in on 40 years. In 2002 it was recognised as a breed by the CKC. More recently, when it was able to fulfill the requirements of the AKC for beginning the process for breed recognition, the membership voted against pursuing this, as they believe the AKC has done irreparable damage to its recognised breeds as showing took precedence over function, health and temperament. Additionally, the mission of the American Mastiff Breeders Council is to produce family pets, not show or working dogs. I realise the CKC is not considered a 'legitimate' registry by many, however it has come a long way in recent years, and is the haven for several proper working breeds who feared the same thing about the AKC.
I was surprised to learn (as I said) that a number of working breeds -- Border Collies, several of the 'gun dogs', and so on opted for the CKC due to the same concerns as the AM community. I would point out that many of our mastiff breeds have only recently been AKC recognised; for example, the Cane Corso was just recognised in 2010. The AM was eligible and met the criteria for beginning the journey a couple of years ago, but elected not to -- for reasons stated.
There are only 8 approved breeders in this country, and they all ONLY sell their puppies with spay/neuter contracts prohibiting breeding. This is specifically to prevent AM's from falling into the hands of BYB's.
It was certainly the founder's objective to create a breed that is identical to the English Mastiff but without the drool, and unfortunately her site still states that. This is one of the points that -- from my observations -- has raised the ire of EM people in terms of potential lost revenues and perceived 'riding on our coattails'. I have several times asked for quantification of the losses or damages incurred -- as my poll shows that AM owners are likely to have EMs as well, hence it is not an 'either/or' purchasing decision. No quantification has been forthcoming, so claims of losses or damage imo are up for conjecture.
Like all more recent breeds, the AM is a work in progress. Thus far, results show that they may tend to have drier mouths, but there is a great deal of individual variation. Health claims are still unsubstantiated. There is evidence that they tend to live longer than EMs -- avg life span appears to be 12 - 14 years, and there are a number of individuals who have lived to 16.
Whilst some believe they do, many forum member-owners I polled/spoke with, said that, in their opinion, the AM does NOT look exactly like the EM, and standards on dog information sites, the American Mastiff Family Forum, and breeders' individual sites reflect these differences -- (i.e., rectangular versus square head, almond eyes, and lighter/leaner builds than EMs). I recently posted a survey on the forum as to what people loved about their AMs, how many they had, and what influenced subsequent purchase decisions after the first. Most people who responded had 2 - 3, as well as EMs. They described their AMs as 'potato chip' dogs -- you couldn't just have one. They liked the 'lighter/leaner' look and longer muzzle of their AMs and described their personality as loving but funny and 'goofy'. Hence, the chief reasons for a purchase of a second or additional AM was not driven by the 'drooless' factor.
Re: Anatolian blood: First of all, we are not talking about a 50 -50 proposition. The founder says 1/8 Anatolian. Whilst I have a bit of an issue about how she arrived at that estimation -- as I don't believe she is a geneticist -- it means fairly small infusion of Anatolian genes. Also, I know what the breed descriptors say, but I have searched for owners of Kangals -- which appears to be the original Anatolian contributor -- to 'interview'. Their comments depict an intelligent dog who is very loving and protective of their family and 'dependents' -- be they sheep, goats, children or neighbors.
I, myself own and EM, whom my family and I absolutely adore! I would encourage you to visit breeders of both EMs and AMs and arrive at the best decision for your family!