I don’t think the question is whether you can learn to do complicated things with either hand. The question is, when the functions of each hand are not identical, and one has a larger motion, higher strength requirement, or a more crucial timing function, which hand will you choose for that function? Yes, in the long run you could almost certainly learn either way, but in the very early stages you will make faster progress if you choose your dominant hand for this function. In my experience, people who quit an instrument often quit early, so maybe getting over that initial hump is easier if you follow the natural preferences of your brain wiring.
On a piano, a keyed woodwind (saxophone, clarinet, etc.), and most percussion instruments (marimbas, etc.) both hands do the same job. There is no reason to reverse the instrument if both hands have the same function.
For many wind instruments with asymmetric functionality, like trumpet, french horn, tuba, or trombone, it is generally the right hand that performs the more energetic action, while the left hand often just holds the instrument, which conforms to the fact that 90% of humans are right-handed. On these instruments the timing is provided by the breath, not the hands, so the choice of which hand to use may be less crucial.
Drum kits are designed for asymmetric functionality, and most kits are set up so that the right foot plays the kick, the right hand plays the hi hat and the left hand plays the snare. Thus the dominant hand keeps time, and most often hits 4 times for every left hand hit. Left handed drummers can easily set their kits up backwards, but doing so makes them less able to sit in with a band on someone else’s kit, which is pretty common for drummers, especially at jam sessions. So, like guitar players, many drummers learn to play right-handed.
There are left-handed violins, but if you want to play in an orchestra, you’ll need to play right-handed so you and your neighbor don’t smack your bows together. Plus it ruins the visual aesthetic of the string section to have one or two people bowing the opposite direction as everyone else. In practice, it is nearly impossible to become a classical performer playing a bowed instrument left-handed. In a smaller, non-classical band, it’s not so much of a problem, but most string players are not self-taught, and most violin (etc.) teachers were classically taught themselves, so most violinists end up playing right-handed.
So guitar players are the special case because the guitar requires asymmetric functionality in the hands, and because guitar instruction is not predominantly classical anymore, not to mention that many rock guitarists are initially self-taught. A left-handed guitarist is free to choose which way to learn, and the only major argument against appeasing the preferences of your brain wiring is the availability of instrument to buy or borrow.
