The founder of judo, Jigoro Kano, recognized this trade-off between "deadly" or severely damaging techniques and our ability to practice these techniques to a useful degree. The situation has improved with technology. Today we have good goggles, MMA and boxing gloves, and steel cups, so if we want to train out ball punches, nukites to the eye, striking, chokes, and grappling, we can do so fairly productively with good gear. But for some moves there is always going to be a trade-off such as you describe: either you practice the technique in a safe manner at full force, developing fluidity and reflexes, or you train it as brutally as possible, omitting the development of a fast, natural reaction for enhanced "realism."
Neil Ohlenkamp, judo 6 dan, describes the paradox in his article, "Fighting or Playing -
The Martial Art vs. Sport Debate", at Fighting Arts:
Sometimes the "combat" arts substitute intellectual perception, a highly subjective and deceptive frame of reference, for genuine training of the body and mind. Some martial arts don't train effectively for self-defense and combat because they can't train for combat without severe risk to training partners. So many martial arts have instead adopted highly stylized, ritualistic, and even dysfunctional training methods. Ironically, martial sports may provide the superior training in effective combat techniques because martial arts can't be practiced in a real life way without injury.
Sport, by removing the lethality, achieves the opposite. That is, sport more typically produces natural, fast, reflexive movement with full power application, achieving a result against a struggling opponent who is also utilizing full power while engaging in strategic and tactical resistance using all of his or her resources and training. Techniques that don't work are soon abandoned, and successful skills are honed against different attackers under a variety of conditions. Maintaining control in various combat situations, both in attack and defense, is difficult when faced with the unpredictable nature of an opponent's efforts, but facing these situations in contest prepares you for similar situations. Each opponent in competition is operating at the limit of physical and psychological skill. By pushing that limit contestants are continually realizing and expanding their potential.
(Emphasis mine.) Judo's answer to this conundrum was to practice the nerfed techniques, such as your calf-to-calf variation, with full power in sparring and competition. Techniques unsuitable for such rigorous, productive training were kept, but relegated to kata-only practice. That is, they would be trained, but only outside of competitive, unpredictable scenarios where the serious injury from those less controllable techniques was likely.
Judo's method was revolutionary at the time. It was--and remains--a systematic method of giving all students copious training against varying degrees of resistance, allowing each student opportunity to develop their skills maximally. Only the techniques that had to be held back were held back. Now, instead of haphazardly producing good fighters out of the naturally athletic students who picked up body skills with ease, judo's method of kihon/uchikomi/kata/randori/shiai consistently turned diligent students, whether naturally athletic or not, into fighters familiar with applying a great many techniques with full power.
True, this new method did not allow for the brutal old-style groin-kicking tomoe-nage to be practiced in randori. But the previous choice was to allow it in free sparring (and see a great many students injured and unable to practice, and a great many more leave or never join for fear of injury) or practice it as a kata, which the judoka did just like the koryu jujitsuka. These students still had fierce and well-tested chokes, trips, hip tosses, arm-locks, sacrifice techniques, foot sweeps, pick-ups, and throws.
I believe this to be the optimal solution. The techniques that work in sparring and competition should be practiced vigorously and at full power as frequently as possible, while techniques that cannot should be practiced occasionally in a proscribed, compliant or semi-compliant sub-optimal manner.