I've heard the argument that the two World Wars could be read, usefully, as merely a second Thirty Years' War. Is this thesis associated with anyone and how does it hold up?
This has been answered elsewhere, as u/CptBuck helpfully pointed out. I will chime in to say I'm confident 500 years from now the First and Second World Wars as well as all the smaller wars, proxy wars, and Cold War associated with the rise and fall of Communism will be taught as one big interconnected multi-generational conflict. There is causation linking one to the next in a chain, and long after we're all dead, historians will have the freedom to say so and connect dots casually that today we want to keep unconnected for the sake of our being so close to each individual tree that the forest is just a rumour spoken of vaguely. There are thousands upon thousands of people who study just one small part of an 80-year span of interconnected global conflicts, but after we're all dead, people will very easily explain how the French experience at Verdun was a factor in their limited role in NATO and insistence in having an independent nuclear deterrent. Those things are connected, of course, but the fellow writing his thesis about Verdun today probably would be deeply uncomfortable saying that battle eventually led to French frogmen sinking Greenpeace's Rainbow Warrior (to use just one example of how academic focuses of today dial back our willingness to generalize).
Not to discourage other answers, but you may want to have a look at previous discussions of the same question here: https://www.reddit.com/r/AskHistorians/search?q=second+thirty+years+war&restrict_sr=on&sort=relevance&t=all
I think this discussion from /u/commustar citing /u/restricteddata hones in on the key question: Is the comparison relevant or useful? Probably not very.