Let's say that Donenfield successfully sued all of his rivals and that Siegel successfully sued Donenfield. What would a world that only knew Superman, the Spectre, Dr. Occult, and Slam Bradley be like?
Well, first let me see if I understand your hypothetical lawsuits. Donenfield drives all other publishers out of the business and then Seigel shuts Donenfield down?
In that case, I give the whole superhero genre about a five-year lifespan and Superman vanishes into the mists of time.
If Seigel had driven National out of the business, he'd have had to start up his own comics publishing firm to keep Superman in print (as in your scenario, the rest are out of business thanks to Harry). I don't think he'd have gotten anywhere. Superman's success story is as much -- perhaps more -- thanks to National's tireless promotion as any artistic merit in the concept itself.
Further, in this hypothetical world where Harry could successfully sue Timely, Fawcett, Charlie Biro, Quality and the rest out of business on the premise that all superheros are an infringement on National's properties,and where Jerry could win the legal argument that Harry's own Batman, Green Lantern and the rest are likewise cribbed from Superman, I hardly think it would end there. Jerry himself would be sued by Philip Wylie, the estate of Edgar Rice Burroughs and Street and Smith for ripping off Gladiator, John Carter of Mars and Doc Savage respectively. And Roy Crane would sue Joe Shuster for stealing his art style.
So even if the concepts had "legs" to last more than a couple of years, the lawyers would have killed comics pretty fast.