I'm thinking about what a rule like this would mean for professional play. So here are my thoughts:
There are two new possibilities with a rule like this: a serve that almost touches the net and a serve that almost goes out. Let's discuss the serve that almost goes out.
Your original post that mentioned super fast serves probably wouldn't happen, simply because long serves aren't as useful as short serves are. Instead, players would probably do the following:
If I were a scumbag, I'd use this rule to go for edge ball serves (specifically, serves that hit the side edge of the table to their backhand). A 70% miss, 30% edge serve is perfectly fine. If I miss the serve, I go back to a normal serve. If I'm successful, the opponent has to deal with the edge ball. This would probably be difficult to punish as long as I aim out rather than aiming in the table, since if it bounces in the table and not on the edge, it just becomes a normal serve. Of course, this scumbag serve can also have spin
Now, let's go to the other extreme. I'd serve as low as possible and as spinny as possible. Same logic. It would be very difficult to return such serves with good quality. If I miss, I go back to a normal serve. This isn't that bad, since pro serves are already pretty low. This rule just incentivizes players to go for the extreme every first serve.
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Here's what I think the effects of the rule will be:
1. As others have said: we'll get a longer game, since we'll have more freebie serves.
2. We'll probably get a world champion who uses edge-ball serves
3. On a more serious note: the increase in missed serves makes the sport look more pathetic.
4. The increase in difficulty to return serves creates less and shorter rallies, making the sport look more pathetic.
5. The increase in pro players missing service return makes the sport look more pathetic.
EDIT:
6. More serves that push the extremes needs to call for better measures to enforce such rules. Cameras should be used to enforce service rules to check if the serve really did touch or net, or if the serve really did hit the edge.