When talking about "alkaline" diets, then have to consider the basic fact that we have very strong natural stomach acid. This actually says enough about "alkaline" diets...
What we need are minerals, most people are deficient of these. Minerals like magnesium, potassium, calcium, sodium, etc. and they of course make an alkaline water solution. Point is if we needed alkalinity the way they claim we do then any kind of substance, even lethal, should be okay to consume, and should compensate the deficiency of whatever mineral. But magnesium does not remove the deficiency of potassium, or any other than magnesium deficiency itself, not to mention toxic stuff.
Those who spout alkalinity nonsense are concentrating on the wrong aspect.
There is something alkaline diets can do. They can alleviate some bad symptoms somewhat. One reason being that minerals make water alkaline, and when eating/drinking something alkaline it transfers that effect over to the body, thus sort of imitating the presence of mineral containing environment, to some extent. But it's no "cure", it's temporary and limited and absolutely does not address nor cure the underlying cause. It's more like fooling the body.