You give the people of the last 1000 years so very little credit as to their ability. And yet, they accomplished feats we would not dream of attempting today. And it is asinine to think that barley, water, yeast, and herbs were so vastly different then than now.
The facts are that beer has ALWAYS been made by steeping mixtures malted barley of varied degree of roast to a point where it creates a syrupy, sugary, soup. And then it is cooled and innoculated either by dropping in the magic beer stick or by a packet of nottingham. Then it is left to sit till it goes foamy and then not. Then it is stored until ready to consume.
The fact is that if the beer was bad, then it would not have become the staple that it had become because there were other options like wine, milk, and fruit juices. It was not simply the only option to water. It was simply the preferred option by many.
What exactly is so special about Crystal malt that you are convinced it was not possible to make 300 year ago? the fact is, nothing. It's green malt, stewed and roasted to a desired doneness. The only part of crystal that is substantial about the patent of it is it specifications of temperature and lovibond. Important for consistent recreation but not imperative. Hell, you can even make the sh!t at home in your oven.
The facts is that brewers today are so anal retentitively caught in the specifics of lovibond, diastatic power, potential yield, percent alpha, and degrees fareighnheit constants that it's MORE amazing that anyone here can produce something drinkable much less appreciable. yet, we are still able to toss our foaming buckets in a closet and result ourselves in crystal clean malty, hoppy goodness.