A couple of remarks:
- The point of such a system wouldn't be 100% accuracy, but 100% consistency. Some margin for error would have to be built in, but everyone would be treated the same
- If it doesn't run and work in real time and signal to the ref within a couple of seconds, it will only add to the current problems with and complaints about having a TMO and calling back the game for earlier infringements. The TMO/VAR gets criticised for taking away the excitement of scoring because you're left waiting for the reviews, often on a very marginal technicality
- It's completely irrelevant to grassroots refs anyway. The Sunday tennis tournament round the corner also doesn't have Hawkeye
From a technical point of view: tennis(/theoretical baseball) systems can't really be compared as examples of the technology needed, as you have a small, unobstructed ball in free flight that has to be analysed in a carefully controlled region. The ball is hit and left to fly in an area where vision is unimpeded, and there's already an objective, concrete performance criterion.
In rugby you may have a centre breaking the line with a (non-spherical) ball tucked under their arm, wrestling through three people and popping it up to a teammate, at any point over a large pitch. Then you have to tell the system whether what it's seeing was a forward pass or not, with 100% certainty - otherwise, it's as fuzzy as the current decisions are.
It's orders of magnitude more complex to train and validate, much less to the accuracy and speed you need. Plus, you need exactly the same camera setup in every international test stadium for consistency; I don't know whether that's possible/practical.
I don't think that's necessarily an odd situation, unless you consider technology paramount over human judgement, and accuracy paramount over other considerations like practicality, enjoyment, viewability.
We could (ad absurdam) institute a rule whereby every time the ref blows up for an infringement, we have to stop and do a TMO review that finds at least three camera angles to confirm the accuracy of the call. You could expect to gain a very marginal increase in accuracy, but the game would be utterly ruined.