Couple of problems with this:
Suppose the spring consists of a strip of, say, high-modulus pre-stressed carbon fiber (substantially lighter and stiffer than wood) - could that not result in harder-hit balls?
Suppose the spring consists of something that yields better directional control at the expense of distance - are there not a bunch of non-power “contact” hitters who’d find that quite useful? (Not every batter swings for the fences with each at-bat.)
The difference in hitting distance between an aluminum and wood bat is being wildly overstated in this thread. There isn’t THAT much difference, and the better and stronger the hitters, the less difference there is. Aluminum bats are lighter, allowing them to be swung faster, but as a result the ball is struck my an object of less mass. A wooden bat swung at the same speed as an aluminum bat of comparable size, but which would weigh four or five ounces lighter, would hit a baseball farther.
Aluminum bats are popular in amateur baseball and softball because they allow a non-professional athlete far more control and bat speed with a bat of the same dimensions. There’s a big difference between a 26-ounce bat and a 32-ounce bat, believe me.
So why doesn’t pro baseball have them? Well, because it violates Rule 1.10(a).
And why haven’t they changed the rule? Well, there’s three reasons:
Even if major league hitters couldn’t hit the ball further, they’d still have more control over the bats, thus increasing swing accuracy and driving batting averages up. Home runs would go up too, just because guys would be hitting the ball more often, and
Because it’s tradition, and
Enforcement. The thing about wooden bats is that enforcing the rules about then is relatively easy; it’s a stick of wood, round, no longer than 42 inches and no thicker than two and 3/4 inches. That’s it.
By comparison, allowing metal bats would require endless tweaking. The term “aluminum bats” actually covers a seemingly endless array of bats that aren’t made out of wood - aluminum, composite, titanium, various alloys. You’ve got single tube, shelled, double wall, and Christ only knows what else. The properties of the bats are often dramatically different. Anyone who’s played serious softball or slo-pitch of amateur baseball knows that leagues will often have dizzying rules concerning what bats are and are not allowed. In my league the league actually has to put authorized seals on any bat to be used in league play because there’s such a ridiculous variety of bats out there that it’s impossible to ask umpires to enforce the rules by remembering them all.
Not only would metal bats create more home runs, but all those broken bat squib hits would probably turn into soft liners over the infield. I’ve read about college pitchers finally realizing they can pitch inside effectively after they go to the minors and the batters are using wood.