I'd say that variety of styles, in and of itself, implies little to nothing about the strength of an era. .
No, that's incorrect, for the very explanation you are about to provide below. What is correct is that it doesn't necessarily imply anything about the strength of any individual player. However, having a number of different styles playing at the highest level absolutely implies a difficulty not present in a homogenized field. To dispute that basic property, one would have to believe that: each player can deal with all style equally and thus individual match-ups won't matter.
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The standard argument is that all Federer had to do to be the top player in the world is make sure that he was the best at the kind of game everyone on tour was playing. Sampras on the other hand had to adjust daily to players with radically different styles - there was no 'one style beats all' formula that he could focus on and perfect in the way Federer did. What people fail to notice is that in Federer's circumstances everyone is really good on every surface and at every event, whereas as in Pete's era the variety of styles resulted in numerous players who, for the most part, were completely inadequate when it came to certain surfaces. Sampras might have the challenge of playing Guga on clay one day, then the relief of a serve and volleyer like Ivanisevic the next. Federer, while not having to cope with so many variables, has to instead contend with players whose styles almost always suit the court they're playing on..
OK, now, just to be clear here, you're presenting two different premises:
1.Sampras had to adjust to radically different styles (not your argument, but you seem to be accepting it here)
2.Federer's opponents are equally good across all surfaces and thus he faces a consistently higher level of opposition
NOW, important to note that the two are NOT related, but you seem to be presenting them as counterbalances.
There are however 3 extreme flaws with your second argument. The first is that Sampras does NOT play the same people on each surface. I think there would be merit to your argument, if say, Sampras played the same 7 people on each surface, but that was not the case. It is not round robin, and it is the nature of the elimination draw that makes it likely that Sampras, or any other top player will actually be playing the toughest opponents on THAT surface (incidently, Ivanisevic on clay, is not a good example, as he was quite adept at all surfaces, and grew up on clay...arguably his grand-slam breakthrough came on clay).
The second problem is that a consistent level of play does not imply the best level of play. To believe this, you would have to believe that all styles do equally well on all surfaces. This is not so. Thus, if you argue, hey, that Sampras had a "break" by playing a guy not suited to a surface, you must also acknowledge that Federer does not face the "challenge" of a facing a guy particularly suited to that surface. This in fact, turns into the very argument that I and others have presented. A lack of challenge through homogenization. However, I do agree with you, that it is possible to get a weaker match-up of this kind! Again, the elimination draw format helps minimize that.
This leads into the third problem, your implication that a homogenized style, and thus, a consistent hierarchy of players, leads to a homogenized level of play. Granted, if two baseliners play the same way they always do, the points are likely to be fairly long, and look fairly similar regardless of the surface, but the ACTUAL level of the play...probably depends on how the "one" style is suited to that surface. For example, I'd say the style is well suited to the FO, I think the average level of play at the FO is higher today...on the other hand, it's WOEFULLY suited to grass (oh please, do not let somebody invoke the more-holes-than-swiss cheese slow grass myth ...to go off on a tangent....lol). In the specific case of Sampras, I'd say his odds of winning the FO in today's era would be worse than ever, on the other hand, I'd say he'd really be cruising through some Wimbledon titles. Now a Nadal or Federer, can give anyone a tussle at Wimbledon or any other surface, but in many of the earlier rounds, Sampras would be about as worried about many of the guys of today as he was facing a Costa, Muster, Kafelnikov etc. In fact, if I came to Federer and said "ok, Roger..it's the third round of Wimbledon...who would you rather face, an in-form xavier malisse or...an in-form Taylor Dent? He'd pick Malisse every day of the week. In fact, if I were Roger, I'd rather face Hewitt than Dent
I'd also take any baseliner out of the top 10 (and maybe some in it) over an in-form Wheaton or Flipper.
In short:
1. a variety of styles at the top level DOES inherently add difficulty to the #1 players challenge
2.you make a good point that the #1 player may get a break at times by facing a person ill-suited to a surface, but I believe this factor would not come close to outweighing either #1, or the converse implication, that a top player must face people particularly suited to a surface
3.again, the elimination draw makes #2 particularly true.