The biggest hitters off the ground right now - Stan and Thiem, aren't that tall. I can't see that shorter players are underpowered from the baseline.
What height helps with so much is serve, moreso because it affects the angle of the serve than leverage. And, in today's game, it's better to be tall with a great serve and mediocre movement than short with good movement.
This is just something that is inherent to the rules of the sport like basketball. If the hoop wasn't 10 feet in the air, NBA players would be as tall as soccer players. And, if tennis players served underhanded like in badminton, the average height in tennis would reflect the average height in that sport.
I would say that the slowing of the courts may be more responsible for this increasing height trend than anything, since it enables poor movement. To be a S&V player back in the day still required a ton of athleticism like Sampras and Tsonga have. You couldn't just be a tall unathletic wide hip'd dude like Raonic or skinny fat guy like Kyrgios.
You have only named two players here. Thiem & Wawrinka & yes, these guys have a lot of topspin to add in with their power, which requires racquet head control, but aside from these two & Verdasco?
Height doesn't only help with the serve. The longer your arms & legs are the easier it is for you to generate power, because you can use your legs as a 'spring-board' and your arms can gear up further. It's the extension of the arm.
'Torque is greater when the size of the moment arm (the distance from the muscle to the joint) is larger. This works with the extension of the arm in the serve. The more the arm is extended, the greater the length of the moment arm and the lever, which in turn puts more acceleration on the racquet head and more power on the ball (see diagram below).
Reference - http://biomechanicstennis-tombuck.we...-momentum.html
^^^^I'll find other supporting articles which support that taller players find it easier to generate power because their arms & legs are longer, but this is a start...
So if you put a guy 6'5 & a guy 5'11 side-by-side, both having the same quality of technique, the 6'5 guy will be able to generate more power. In Alex Zverev's case, because he is also a good athlete, it's double the pleasure, because now he can move well enough to prepare early for shots & hit them hard. So the edge the 5'11 guy has on the 6'5 guy, from being able to move better, has suddenly been eradicated.
A good tall player will always beat an equally good small player, in modern tennis. Guys like Goffin and Nishikori have to play up along the baseline, unless they are on clay & take the ball early & aggress & when they are on the backfoot & in a defensive position, counter-punch. Especially Goffin, who can't generate power for shit.
Some make the argument that as a small player, your technique will be more compact & reliable/efficient, but if we look at Zverev & Fritz's backhands, they're also compact.