This seems to be a fair appraisal, however, when the world number two is a 120 pound war veteran, who laboured and lost in long matches, and he was succeeded at number two by another small player who weakened and retired at the age of 26, you must admit that this is one weak field, the weakest ever.
No that does not follow at all. Lacoste retired early, at 26, but one thing to keep in mind is how early he matured as a player (he was 22 when he reached #1). Also, HL Doherty had retired early too, reaching #1 at age 27 and retiring by age 31, while still #1. And he retired due to sickness as well -- tuberculosis, again, which was such a great problem back then.
If you think that the Lacoste era was weak because Lacoste had health problems, then certainly you have to say the same for the era in which Doherty was #1.
You seem to be judging these eras according to later standards, not just when it comes to sickness but also height. People were smaller back then, and that has to be taken into consideration when considering Bill Johnston. It should not be a surprise that the smallest champion of all time (if that's what Johnston was) played almost a century ago, because people were smaller then.
In all the contemporary reports from the Tilden era, Johnston is recognized as a small man but no one implies that they're living in a weak era simply because Johnston is a top player. To the contrary Johnston is compared favorably with the greatest champions of the past, and there is only admiration for just how well he hit the ball -- and particularly how hard he hit his forehand. He was not a player who waited for his opponent's errors; he went for powerful shots and winners. One report after another observes how he manages to use his relatively small frame to maximum effect -- which if you recall was something routinely said about Justine Henin.
At Forest Hills in 1915, Johnston defeated Maurice McLoughlin by putting him on the defensive, despite all the power of "the Comet." Read this appraisal of Johnston's forehand in the New York Times and tell me if they were dismissing Johnston as a small man:
"Probably never in all the years of the historic All Comers has a player displayed such phenomenal command of the ball with a forehand stroke. There were many competent judges present yesterday who declared that its equal was not to be found on the courts anywhere, and it is likely that a representative number of players, including R. Norris Williams, 2d, the playing through title holder who lost to it in the semifinal, Karl H. Behr and Harold H. Hackett, would willingly indorse [sic] the statement...."
And after the 1919 final in which Johnston defeated Tilden:
"The forehand drive of William M. Johnston is unquestionably the greatest single tennis shot in the world, bar none. He seems able to use it with every possible degree of speed, with an accuracy that baffles the fastest court covering, and with a steadiness which has discouraged every opponent he has ever faced. No stroke has ever been developed by any other player to equal its efficiency and general dependability."
And their appraisal of Johnston's volleying ability, after his five-set loss to Tilden in 1920:
"Here he was distinctly Tilden’s superior. Not only was he able to volley balls that would have knocked down an ordinary player, and to reach low, well-placed passing shots that could scarcely be seen in their flight, but he turned a majority of these extraordinary gets into actual factors in his attack, either scoring outright or forcing a defensive position which ultimately brought him the point."
I think the problem you're facing here is that no one who observed Tilden's tennis back then, to my knowledge, felt that they were looking at a step backward in the game, or at an inferior champion. Quite to the contrary. And Johnston, for a #2 player, was held in the highest regard. I think the burden of proof is on the argument that Tilden's brand of tennis was inferior to what had come before. Merely pointing to one of Tilden's rivals who was a short man and another who became infirm does not show this, particularly in the context of that time period when people were smaller and were more often ill or infirm.
Seriously, this is the first time that I've seen someone point to the French Musketeers -- one of the greatest national teams ever, and a unique concentration of talent -- and call it a weak era.
Further, with the advancement of the game after WW I, players appear to have matured at a younger age, about 21 or so, which has been the standard ever since, and played more international matches.
I don't know if this is true about maturation at a younger age; I'd need to see statistical proof. However, if it is true, then that means that you are judging Tilden's maturation as a tennis player according to later standards. And that would contradict your earlier argument that Tilden's maturation at age 27 is something uniquely out of step with the course of tennis history. Maybe it was, in the context of its era, not nearly as unique as you think.