A lot of people love the taste of chemical fake food these days, even much prefer it to the original article.
The above-mentioned plain and salt-and-tiny-amount-of-butter-only flakes can be turned into a reasonable semblance of mashed potatoes. Neither is the equal of the, alas, now discontinued Dona Inez instant criolla potato granules made in Peru. Those convinced me it is possible to produce really good instant potatoes; it just isn't done by the mega food corporations that are generally the only suppliers. All the added flavorings in most offerings are not only for convenience, but to disguise how ****-poor the actual instant potato flavor is in those.
I agree with you that the (plain dehydrated) shredded hash browns* are probably the best of the dehydrated storage potato offerings and also cook up quicker than any other form. Dehydrated precooked papa seca are also good, although you need to remember to set those to soak in cold water overnight or for the day if you want to cook them up in 5 minutes. Either cooked/rehydrated and run through a food mill can make mashed potatoes or add some flavor and interest to the instant flakes.
ETA: Canned potatoes soak up some extra water, so if trying to use those for mashed, make up your milk to only evaporated or even double evaporated strength to keep from ending up with a watery mess.
* However, any seasoned hash browns with added oil and other ingredients will not fare better in storage than similarly doctored up potato flakes. Those Idahoan hash browns in cartons have this ingredient list:
Idaho® potatoes, sunflower oil, salt, dextrose, onion powder, monoglycerides, calcium stearoyl lactylate. Freshness preserved by sodium acid pyrophosphate and sodium bisulfite.
Like the seasoned potato flakes, these are not suitable for anything other than short-term pantry storage (assuming you don't object to the ingredient list). And one of the reasons so many people love them is that, as with so much other highly processed food these days, they have sugar added as well as oil and salt.
When posting here, I sometimes forget what adulterated fast-food forms of things exist because I just never buy or eat such. You can buy dehydrated hash browns that are just potatoes with sodium bisulfite to keep them white when dried. You can also make your own papa seca version by precooking before shredding, which avoids the need of anything to keep them from turning brown in the dehydrator. I think plain dehydrated hash browns are the best form of dehydrated storage potato on offer, just as I think unadulterated lard is one of the best storage fats. However, that doesn't mean the form of either you will find in your supermarket is what I'm talking about.