First, I must say that while I have posted very little on the board I "visit" quite frequently and thoroughly enjoy reading the various posts/contributions - particularly those by the various instructors and "students" of the game. But, truth be told, this modern versus traditional thing is a bit silly. Wegner and the MTM guys are marketing a product (which I personally don't believe will ultimately help very many players advance far beyond hitting a nice rally ball from the baseline) with Oscar himself pushing his own "history" as a selling point a bit too hard. John, as well as some of the anti-mtm-at-all-costs guys, ultimately just seem to egg them on, however. Whether that it due to their pushing a competing set of instructional videos/websites or because they really disagree with the fundamentally different approaches presented is sometimes is a bit difficult to discern.
While the only thing I can point to as "credentials" are my sort of "nerdy" love of a game that I have played for over thirty years (my first "good" racquet was a Wilson Kramer Pro Staff) and a little teaching experience, I would like to offer the following: "modern" technique (like others have already indicated) is merely evidence of the evolution in the way the game is played basically due to three, what I think are overwhelming obvious factors - 1) the now-favored use of the semi-western and western forehand grip among both professional and recreational players; 2) the speed of the game as influenced by better athletes at the higher levels and changes in racquet/string technology; and 3) the increasing uniformity of surfaces on which the game is played.
When I was a kid, players like Borg, Vilas, Clerc, Solomon, Arias, etc. all hit with semi-open to open stance forehands. Why is that significant? Well, all of them played with semi-western to western forehand grips which made it very difficult for them to make good contact with the ball from a close-stance position as they had to hit it so much further out in front of their body compared to those who employed eastern or continental grips. Initially, this approach to the forehand seemed to be confined more to the so-called clay-court specialists of the day who could sit back on the baseline and, while taking a big cut at the ball, hit topspin forehands landing around the service line that would often bounce above the heads of their opponents on the opposite side of the court. Borg (and a few others) with the more semi-western grips, however, could also flatten their stroke out pretty effectively when necessary and thus hold their own on faster surfaces as well. It was this ability to hit both with more, naturally-generated topspin (due to the swing path necessitated by the grip) as well as to flatten the ball out when needed that heralded the significant rise in popularity of the semi-western forehand. Probably one of the best examples of this shift to the becoming-more-prominent semi-western and open to semi-open stance forehand and also arguably of the advent of the "modern" power-baseline game among the pros was seen with the rise of Ivan Lendl to the top tier of the sport. Lendl's hammer of a forehand, with his semi-western grip and abbreviated elbow-first small loop take-back, made him a dominant force in the game and influenced the technique of many future hall-of-fame players who followed including, among others, Pete Sampras. As the semi-western grip became more common among the pros, it naturally began to supplant the teaching of the traditional eastern forehand at the recreational level and, by extension, the closed-stance approach. Now, by this time we also have graphite, larger headed racquets starting to relegate wood frames to the dustbin of history. These larger but less cumbersome racquets made it easier to hit the ball as well as generate pace and, by extension, the speed of the game began to change. So now you have people both hitting the ball harder and having less time to set-up (as the ball is coming at them at a higher speed), making it even more necessary for them to generate additional spin to keep the ball in play and do so, timing-wise, from a more open stance. Fast-forward a couple of years and slower, more uniform surfaces have made it nearly impossible to serve/volley on the pro tours as it is now so much easier to rip passing shots off just about anything other than the biggest serve or perfect approach. More extreme western grips are also now being used by significantly more players - primarily to generate more topspin so that they can keep the ball in play and increasingly swing harder. At least at the professional level, it has become a game of trying to push the opponent as far behind the baseline or from the center of the court as possible so one can smack an acute angle winner or drive the ball through the open court. No more finesse (aside from the occasional drop shot), very little variety, and bigger, faster, stronger competitors with larger, more powerful equipment, plying their trade on surfaces that perpetuate that mode of play. Evolution.
Okay - got that off my chest
Now, the problem with approaches like Oscar's is how it tries to teach amateurs to "play like the pros" when most rec players do not have the strength or skill to ever be able to generate the amount of racquet-head speed that the pros hit with - whether they are pulling/jerking across or hitting through the ball toward the target. As such, instead of producing a heavy, penetrating topspin forehand the student ends up with spinny, lower paced rally ball that sits up a little too much. This kind of approach to the game ignores the fact that it is still necessary to teach/learn how to hit through the ball while moving forward in order to reach a more advanced level of play. It is absurd to suggest that at the professional level (whether ATP or WTA) or the highest amateur levels that players do not still, at least on occasion, hit through the ball with forward momentum/movement. To say otherwise would actually amount to approaching the game in a more one-dimensional way than the more traditional teaching methods that Oscar and the mtm pundits deride. With but a few exceptions, almost everyone who plays at the higher levels of the game at least occasionally still hits an "semi" old school" forehand like a Borg/Lendl/Sampras. That fact, and the history/reason for development of same, should be taken into account when teaching the game today. Bottom line is that you can't have one without the other as they are linked/tied to each other in such a way that makes its silly to say one can draw a direct distinction between them and still succeed in teaching/learning this great sport.
Thank you for putting up with my rant. Happy New Year!