I think you all are overthinking this a lot. Players hit 2nd serves as hard or soft as they do based on how confident they are that they can land a very high % of them. Murray has an especially terrible 2nd serve because he hits it soft and also misses a fair share of them. This just indicates he has no confidence in being able to land a good % if he hit it harder. Federer's 2nd serve is certainly above average but there is a reason he isn't going for 110 or higher on 2nd serves. And that's because in his estimation he would miss too many of them for it to outweigh the benefit of a bigger 2nd serve. It is still a game of math.
If the players are only given one serve then they are going to use their 2nd serves for the most part. But what that rule change would do is affect how much a player practiced his serve. If players practice hitting 120 on 1st and 100 on 2nd serves they'd likely start practice just hitting 105-110. They'd totally forget serving big and then maybe try to practice the serve to the point where they can add a few MPH from their second serve
Maybe we're overthinking, or you're not using enough imagination. I think everyone understands how players basically approach their 2nd vs. their 1st. But we're talking about a world in which there is only one. I'd say it's not a game of math, it's a parallel universe. You've (rightfully, I think) said before that it poorly impacts a player with a weak second serve. A poor second serve is a lack of confidence, I'd say. Murray, by accounts of British journalists, hits his spots well on serves in practice. In match play, he tends to revert to an under-confident and weak second. I think he'd be one to be ill-served by such a rule change.
I don't think you're trying hard enough to image the rejiggering of the thinking, though. Early in a serving game, a player would try to go for more, IMO. Someone like Roger, for example, who wins on placement and not on MPHs, could feel good about going for it early in the service game. If it goes poorly, then the caution would come in. If it's going well, however, the player would be emboldened to keep going for it, right? But you are wrong, imho, to assume that EVERY serve would revert to a second serve. The calculations would be different. And good servers would still be good servers, though they'd likely win fewer service games. I do understand that this particular innovation would benefit the returner.
I'm not sure I agree with you about what they'd practice. I still think a big server would practice, and employ, the big serve. I just think it would affect where they use them in the game. As Mrzz says: they don't just close their eyes and serve a bomb. They are professionals, and have some trust in their shots.
