Contrary to what you may believe, Sniper does not have consistent counterplay. Everything you suggest is contingent purely on the Sniper's own fallibility, which is poor game design. Strafing erratically, duck jumping, twisting around, rocket jumping, and airstrafing are not Sniper counters; those techniques are tantamount to standing your ground and making noise to ward off a bear attack. Ultimately, your survival is predicated on the bear's audacity, in much the same way your survival against Sniper is predicated purely on his ability to place his crosshair over your model. Sniper as a class removes player agency due the fact that your input in the "encounter" is meaningless--only his input matters. Spy is not a Sniper counter, as any Sniper that's aware of his surroundings will position himself in area that makes predicting their approach vectors rather simplistic. One shot is all that's required to remove Spy as a threat once detected, and the Razorback obviates the danger posed by backstabs.
This has been debated endlessly in the upper echelons of the competitive TF2 community and elsewhere, and even b4nny himself agrees that Sniper requires major alterations. Suffice it to say, in the case of Hightower, the only limiting factor placed on Sniper's efficacy is cover, and Hightower does not provide an ample amount of this. Sniper has free reign to shut down 90% of the map, and can acquire sightlines to any conceivable area. When you have five or more Snipers on the server, it becomes so locked down that absolutely no one has any enjoyment whatsoever, and more often than not, the server begins to empty out as a result. This was the same reason we limited sentries years ago.